Implement Discard and EnsureBackbufferCHROMIUM using command buffer instead of IPC.

content/common/gpu/gpu_messages.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Multiply-included message file, hence no include guard here, but see below
 // for a much smaller-than-usual include guard section.
 
 #include <vector>
 #include <string>
 
@@ skipping 428 matching lines @@
 // delayed.
 IPC_MESSAGE_ROUTED1(GpuCommandBufferMsg_Echo,
                     IPC::Message /* reply */)
 
 // Response to a GpuChannelMsg_Echo message.
 IPC_MESSAGE_ROUTED0(GpuCommandBufferMsg_EchoAck)
 
 // Send to stub on surface visibility change.
 IPC_MESSAGE_ROUTED1(GpuCommandBufferMsg_SetSurfaceVisible, bool /* visible */)
 
-IPC_MESSAGE_ROUTED0(GpuCommandBufferMsg_DiscardBackbuffer)
-IPC_MESSAGE_ROUTED0(GpuCommandBufferMsg_EnsureBackbuffer)
-
 // Sent to proxy when the gpu memory manager changes its memory allocation.
 IPC_MESSAGE_ROUTED1(GpuCommandBufferMsg_SetMemoryAllocation,
                     GpuMemoryAllocationForRenderer /* allocation */)
 
 //------------------------------------------------------------------------------
 // Accelerated Video Decoder Messages
 // These messages are sent from Renderer process to GPU process.
 
 // Send input buffer for decoding.
 IPC_MESSAGE_ROUTED3(AcceleratedVideoDecoderMsg_Decode,
@@ skipping 49 matching lines @@
 
 // Confirm decoder has been flushed.
 IPC_MESSAGE_ROUTED0(AcceleratedVideoDecoderHostMsg_FlushDone)
 
 // Confirm decoder has been reset.
 IPC_MESSAGE_ROUTED0(AcceleratedVideoDecoderHostMsg_ResetDone)
 
 // Video decoder has encountered an error.
 IPC_MESSAGE_ROUTED1(AcceleratedVideoDecoderHostMsg_ErrorNotification,
                     uint32) /* Error ID */