Add support for absl::string_view to flatbuffers

This lets us use string_views to manipulate flatbuffers::String objects
much easier.

Change-Id: Ie51864822af0cd8209d643767d6f73a8d7e025f8
diff --git a/third_party/flatbuffers/include/flatbuffers/base.h b/third_party/flatbuffers/include/flatbuffers/base.h
index d98da13..4eff0a6 100644
--- a/third_party/flatbuffers/include/flatbuffers/base.h
+++ b/third_party/flatbuffers/include/flatbuffers/base.h
@@ -212,6 +212,13 @@
         typedef std::experimental::string_view string_view;
       }
       #define FLATBUFFERS_HAS_STRING_VIEW 1
+    // Check for absl::string_view (in c++14, compiler-dependent)
+    #elif __has_include("absl/strings/string_view.h")
+      #include "absl/strings/string_view.h"
+      namespace flatbuffers {
+        typedef absl::string_view string_view;
+      }
+      #define FLATBUFFERS_HAS_STRING_VIEW 1
     #endif
   #endif // __has_include
 #endif // !FLATBUFFERS_HAS_STRING_VIEW