Add MGLEcho

gpu/command_buffer/client/gles2_implementation.h

Index: gpu/command_buffer/client/gles2_implementation.h
diff --git a/gpu/command_buffer/client/gles2_implementation.h b/gpu/command_buffer/client/gles2_implementation.h
index 31dc7e4a3b9d23ba9be5e369e7353cf53f9df90d..b03faff7c09b2604fc992fe358ebf72b8a38c94d 100644
--- a/gpu/command_buffer/client/gles2_implementation.h
+++ b/gpu/command_buffer/client/gles2_implementation.h
@@ -247,6 +247,9 @@ class GLES2_IMPL_EXPORT GLES2Implementation
   void FreeUnusedSharedMemory();
   void FreeEverything();
 
+  // TODO(abarth): This should probably be part of ContextSupport.
+  void Echo(const base::Closure& callback);
+
   // ContextSupport implementation.
   void SignalSyncPoint(uint32 sync_point,
                        const base::Closure& callback) override;