Implement Discard and EnsureBackbufferCHROMIUM using command buffer instead of IPC.

content/common/gpu/gpu_command_buffer_stub.cc

Index: content/common/gpu/gpu_command_buffer_stub.cc
diff --git a/content/common/gpu/gpu_command_buffer_stub.cc b/content/common/gpu/gpu_command_buffer_stub.cc
index 6d92774ef954b76d22ce95dd44f8cce85fc5f59f..0a01a3b5a9830bb70adef3a72b0af9b3484c49b2 100644
--- a/content/common/gpu/gpu_command_buffer_stub.cc
+++ b/content/common/gpu/gpu_command_buffer_stub.cc
@@ -123,10 +123,6 @@ bool GpuCommandBufferStub::OnMessageReceived(const IPC::Message& message) {
                         OnDestroyVideoDecoder)
     IPC_MESSAGE_HANDLER(GpuCommandBufferMsg_SetSurfaceVisible,
                         OnSetSurfaceVisible)
-    IPC_MESSAGE_HANDLER(GpuCommandBufferMsg_DiscardBackbuffer,
-                        OnDiscardBackbuffer)
-    IPC_MESSAGE_HANDLER(GpuCommandBufferMsg_EnsureBackbuffer,
-                        OnEnsureBackbuffer)
     IPC_MESSAGE_UNHANDLED(handled = false)
   IPC_END_MESSAGE_MAP()
 
@@ -309,6 +305,12 @@ void GpuCommandBufferStub::OnInitialize(
   decoder_->SetMsgCallback(
       base::Bind(&GpuCommandBufferStub::SendConsoleMessage,
                  base::Unretained(this)));
+  decoder_->SetDiscardBackbufferCallback(
+      base::Bind(&GpuCommandBufferStub::OnDiscardBackbuffer,
+                 base::Unretained(this)));
+  decoder_->SetEnsureBackbufferCallback(
+      base::Bind(&GpuCommandBufferStub::OnEnsureBackbuffer,
+                 base::Unretained(this)));
 
   command_buffer_->SetPutOffsetChangeCallback(
       base::Bind(&gpu::GpuScheduler::PutChanged,