Added GPU process "echo" IPC message.

content/common/gpu/gpu_messages.h

Index: content/common/gpu/gpu_messages.h
===================================================================
--- content/common/gpu/gpu_messages.h	(revision 98655)
+++ content/common/gpu/gpu_messages.h	(working copy)
@@ -307,6 +307,10 @@
                      int32, /* context_route_id */
                      int32 /* host_id */)
 
+// Request that the GPU process reply with the given message.
+IPC_MESSAGE_CONTROL1(GpuChannelMsg_Echo,
+                     IPC::Message /* reply */)
+
 //------------------------------------------------------------------------------
 // GPU Command Buffer Messages
 // These are messages between a renderer process to the GPU process relating to
@@ -357,9 +361,6 @@
 IPC_MESSAGE_ROUTED1(GpuCommandBufferMsg_UpdateState,
                     gpu::CommandBuffer::State /* state */)
 
-// Indicates that a SwapBuffers call has been issued.
-IPC_MESSAGE_ROUTED0(GpuCommandBufferMsg_SwapBuffers)
-
 // Create a shared memory transfer buffer. Returns an id that can be used to
 // identify the transfer buffer from a comment.
 IPC_SYNC_MESSAGE_ROUTED2_1(GpuCommandBufferMsg_CreateTransferBuffer,
@@ -418,6 +419,9 @@
 IPC_MESSAGE_ROUTED1(GpuCommandBufferMsg_Destroyed,
                     gpu::error::ContextLostReason /* reason */)
 
+// Response to a GpuChannelMsg_Echo message.
+IPC_MESSAGE_ROUTED0(GpuCommandBufferMsg_EchoAck)
+
 // --------------------------------------------------------------------------
 // TransportTexture messages
 //