Add WebGl2 benchmarking example

This makes it so that we can specify a custom HTML shell file for the
emcc_binary rule.

Change-Id: I919e917d6fd92fed6ab77775c5373d2886edacca
diff --git a/build_tests/minimal_shell.html b/build_tests/minimal_shell.html
new file mode 100644
index 0000000..6158571
--- /dev/null
+++ b/build_tests/minimal_shell.html
@@ -0,0 +1,128 @@
+<!doctype html>
+<!--This file is adapted from https://github.com/emscripten-core/emscripten/blob/incoming/src/shell_minimal.html-->
+<html lang="en-us">
+  <head>
+    <meta charset="utf-8">
+    <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+    <title>Emscripten-Generated Code</title>
+    <style>
+      .emscripten { padding-right: 0; margin-left: auto; margin-right: auto; display: block; }
+      div.emscripten { text-align: center; }
+      /* the canvas *must not* have any border or padding, or mouse coords will be wrong */
+      canvas.emscripten {
+        border: 0px none;
+        background-color: black;
+      }
+
+      .spinner {
+        height: 50px;
+        width: 50px;
+        margin: 0px auto;
+        -webkit-animation: rotation .8s linear infinite;
+        -moz-animation: rotation .8s linear infinite;
+        -o-animation: rotation .8s linear infinite;
+        animation: rotation 0.8s linear infinite;
+        border-left: 10px solid rgb(0,150,240);
+        border-right: 10px solid rgb(0,150,240);
+        border-bottom: 10px solid rgb(0,150,240);
+        border-top: 10px solid rgb(100,0,200);
+        border-radius: 100%;
+        background-color: rgb(200,100,250);
+      }
+      @-webkit-keyframes rotation {
+        from {-webkit-transform: rotate(0deg);}
+        to {-webkit-transform: rotate(360deg);}
+      }
+      @-moz-keyframes rotation {
+        from {-moz-transform: rotate(0deg);}
+        to {-moz-transform: rotate(360deg);}
+      }
+      @-o-keyframes rotation {
+        from {-o-transform: rotate(0deg);}
+        to {-o-transform: rotate(360deg);}
+      }
+      @keyframes rotation {
+        from {transform: rotate(0deg);}
+        to {transform: rotate(360deg);}
+      }
+
+    </style>
+  </head>
+  <body>
+    <figure style="overflow:visible;" id="spinner"><div class="spinner"></div><center style="margin-top:0.5em"><strong>emscripten</strong></center></figure>
+    <div class="emscripten" id="status">Downloading...</div>
+    <div class="emscripten">
+      <progress value="0" max="100" id="progress" hidden=1></progress>
+    </div>
+    <!--The width and height values in the canvas specify the pixel width/height for the WebGL canvas.
+        The actual on-screen size is controlled by the stylesheet.-->
+    <canvas class="emscripten" id="canvas" oncontextmenu="event.preventDefault()" width=3000 height=2000></canvas>
+
+    <script type='text/javascript'>
+      var statusElement = document.getElementById('status');
+      var progressElement = document.getElementById('progress');
+      var spinnerElement = document.getElementById('spinner');
+
+      var Module = {
+        preRun: [],
+        postRun: [],
+        print: (function() {
+          return function(text) {
+            if (arguments.length > 1) text = Array.prototype.slice.call(arguments).join(' ');
+            console.log(text);
+          };
+        })(),
+        printErr: function(text) {
+          if (arguments.length > 1) text = Array.prototype.slice.call(arguments).join(' ');
+          console.error(text);
+        },
+        canvas: (function() {
+          var canvas = document.getElementById('canvas');
+
+          // As a default initial behavior, pop up an alert when webgl context is lost. To make your
+          // application robust, you may want to override this behavior before shipping!
+          // See http://www.khronos.org/registry/webgl/specs/latest/1.0/#5.15.2
+          canvas.addEventListener("webglcontextlost", function(e) { alert('WebGL context lost. You will need to reload the page.'); e.preventDefault(); }, false);
+
+          return canvas;
+        })(),
+        setStatus: function(text) {
+          if (!Module.setStatus.last) Module.setStatus.last = { time: Date.now(), text: '' };
+          if (text === Module.setStatus.last.text) return;
+          var m = text.match(/([^(]+)\((\d+(\.\d+)?)\/(\d+)\)/);
+          var now = Date.now();
+          if (m && now - Module.setStatus.last.time < 30) return; // if this is a progress update, skip it if too soon
+          Module.setStatus.last.time = now;
+          Module.setStatus.last.text = text;
+          if (m) {
+            text = m[1];
+            progressElement.value = parseInt(m[2])*100;
+            progressElement.max = parseInt(m[4])*100;
+            progressElement.hidden = false;
+            spinnerElement.hidden = false;
+          } else {
+            progressElement.value = null;
+            progressElement.max = null;
+            progressElement.hidden = true;
+            if (!text) spinnerElement.hidden = true;
+          }
+          statusElement.innerHTML = text;
+        },
+        totalDependencies: 0,
+        monitorRunDependencies: function(left) {
+          this.totalDependencies = Math.max(this.totalDependencies, left);
+          Module.setStatus(left ? 'Preparing... (' + (this.totalDependencies-left) + '/' + this.totalDependencies + ')' : 'All downloads complete.');
+        }
+      };
+      Module.setStatus('Downloading...');
+      window.onerror = function() {
+        Module.setStatus('Exception thrown, see JavaScript console');
+        spinnerElement.style.display = 'none';
+        Module.setStatus = function(text) {
+          if (text) Module.printErr('[post-exception status] ' + text);
+        };
+      };
+    </script>
+    {{{ SCRIPT }}}
+  </body>
+</html>