const-qualify Context::data

Makes it harder for people to accidentally write through it, which they
shouldn't be.

Change-Id: Icea11e9090e01813e3e7b4a15aad601d8b5478c7
diff --git a/aos/events/event_loop.h b/aos/events/event_loop.h
index 81dfe42..b96f25e 100644
--- a/aos/events/event_loop.h
+++ b/aos/events/event_loop.h
@@ -57,7 +57,7 @@
   // Size of the data sent.
   size_t size;
   // Pointer to the data.
-  void *data;
+  const void *data;
 };
 
 // Raw version of fetcher. Contains a local variable that the fetcher will
diff --git a/aos/events/event_loop_param_test.cc b/aos/events/event_loop_param_test.cc
index dc7c5d0..087984d 100644
--- a/aos/events/event_loop_param_test.cc
+++ b/aos/events/event_loop_param_test.cc
@@ -933,8 +933,8 @@
     EXPECT_GE(loop1->context().realtime_event_time + chrono::milliseconds(500),
               realtime_now);
 
-    EXPECT_LT(&msg, reinterpret_cast<void *>(
-                        reinterpret_cast<char *>(loop1->context().data) +
+    EXPECT_LT(&msg, reinterpret_cast<const void *>(
+                        reinterpret_cast<const char *>(loop1->context().data) +
                         loop1->context().size));
     triggered = true;
   });
@@ -967,8 +967,8 @@
   // Confirm that the data pointer makes sense.
   EXPECT_GT(fetcher.get(), fetcher.context().data);
   EXPECT_LT(fetcher.get(),
-            reinterpret_cast<void *>(
-                reinterpret_cast<char *>(fetcher.context().data) +
+            reinterpret_cast<const void *>(
+                reinterpret_cast<const char *>(fetcher.context().data) +
                 fetcher.context().size));
   EXPECT_TRUE(monotonic_time_offset < ::std::chrono::milliseconds(500))
       << ": Got "
@@ -1052,8 +1052,8 @@
   // Confirm that the data pointer makes sense.
   EXPECT_GT(fetcher.get(), fetcher.context().data);
   EXPECT_LT(fetcher.get(),
-            reinterpret_cast<void *>(
-                reinterpret_cast<char *>(fetcher.context().data) +
+            reinterpret_cast<const void *>(
+                reinterpret_cast<const char *>(fetcher.context().data) +
                 fetcher.context().size));
   EXPECT_TRUE(monotonic_time_offset < ::std::chrono::milliseconds(500))
       << ": Got "
diff --git a/aos/events/logging/logfile_utils.cc b/aos/events/logging/logfile_utils.cc
index 2cc2f61..123c579 100644
--- a/aos/events/logging/logfile_utils.cc
+++ b/aos/events/logging/logfile_utils.cc
@@ -113,8 +113,8 @@
     case LogType::kLogMessage:
     case LogType::kLogMessageAndDeliveryTime:
     case LogType::kLogRemoteMessage:
-      data_offset =
-          fbb->CreateVector(static_cast<uint8_t *>(context.data), context.size);
+      data_offset = fbb->CreateVector(
+          static_cast<const uint8_t *>(context.data), context.size);
       break;
 
     case LogType::kLogDeliveryTimeOnly:
diff --git a/aos/network/web_proxy_utils.cc b/aos/network/web_proxy_utils.cc
index dbcc2e6..15aedf2 100644
--- a/aos/network/web_proxy_utils.cc
+++ b/aos/network/web_proxy_utils.cc
@@ -19,13 +19,13 @@
   flatbuffers::Offset<flatbuffers::Vector<uint8_t>> data_offset;
   if (kPacketSize * (packet_index + 1) < context.size) {
     data_offset = fbb->CreateVector(
-        static_cast<uint8_t *>(context.data) + kPacketSize * packet_index,
+        static_cast<const uint8_t *>(context.data) + kPacketSize * packet_index,
         kPacketSize);
   } else {
     int prefix_size = kPacketSize * packet_index;
-    data_offset =
-        fbb->CreateVector(static_cast<uint8_t *>(context.data) + prefix_size,
-                          context.size - prefix_size);
+    data_offset = fbb->CreateVector(
+        static_cast<const uint8_t *>(context.data) + prefix_size,
+        context.size - prefix_size);
   }
 
   MessageHeader::Builder message_header_builder(*fbb);