Implement Discard and EnsureBackbufferCHROMIUM using command buffer instead of IPC.

gpu/command_buffer/service/gles2_cmd_decoder.h

Index: gpu/command_buffer/service/gles2_cmd_decoder.h
diff --git a/gpu/command_buffer/service/gles2_cmd_decoder.h b/gpu/command_buffer/service/gles2_cmd_decoder.h
index 1d1277ba7eaa579f4cb8797244fbef5ef93041e6..4c2e162225d6bc15cfeb3e7a4341075303c36645 100644
--- a/gpu/command_buffer/service/gles2_cmd_decoder.h
+++ b/gpu/command_buffer/service/gles2_cmd_decoder.h
@@ -45,6 +45,8 @@ class GPU_EXPORT GLES2Decoder : public CommonDecoder {
  public:
   typedef error::Error Error;
   typedef base::Callback<void(int32 id, const std::string& msg)> MsgCallback;
+  typedef base::Callback<void()> DiscardBackbufferCallback;
+  typedef base::Callback<void()> EnsureBackbufferCallback;
 
   // Creates a decoder.
   static GLES2Decoder* Create(ContextGroup* group);
@@ -164,6 +166,12 @@ class GPU_EXPORT GLES2Decoder : public CommonDecoder {
   // A callback for messages from the decoder.
   virtual void SetMsgCallback(const MsgCallback& callback) = 0;
 
+  // Called when discard/ensure backbuffer messages are decoded.
+  virtual void SetDiscardBackbufferCallback(
+      const DiscardBackbufferCallback& callback) = 0;
+  virtual void SetEnsureBackbufferCallback(
+      const EnsureBackbufferCallback& callback) = 0;
+
   static bool IsAngle();
 
   // Used for testing only