Added GPU process "echo" IPC message.

gpu/command_buffer/service/gpu_scheduler.h

Index: gpu/command_buffer/service/gpu_scheduler.h
===================================================================
--- gpu/command_buffer/service/gpu_scheduler.h	(revision 98655)
+++ gpu/command_buffer/service/gpu_scheduler.h	(working copy)
@@ -127,17 +127,17 @@
   virtual uint64 GetSurfaceId();
 
   void DidDestroySurface();
+
+  // Sets a callback which is called when a SwapBuffers command is processed.
+  // Must be called after Initialize().
+  // It is not defined on which thread this callback is called.
+  void SetSwapBuffersCallback(Callback0::Type* callback);
 #endif
 
   // Sets a callback that is called when a glResizeCHROMIUM command
   // is processed.
   void SetResizeCallback(Callback1<gfx::Size>::Type* callback);
 
-  // Sets a callback which is called when a SwapBuffers command is processed.
-  // Must be called after Initialize().
-  // It is not defined on which thread this callback is called.
-  void SetSwapBuffersCallback(Callback0::Type* callback);
-
   void SetCommandProcessedCallback(Callback0::Type* callback);
 
   // Get the GLES2Decoder associated with this scheduler.
@@ -149,9 +149,11 @@
                gles2::GLES2Decoder* decoder,
                CommandParser* parser);
 
+#if defined(OS_MACOSX)
   // Called via a callback just before we are supposed to call the
   // user's swap buffers callback.
   void WillSwapBuffers();
+#endif
 
   // The GpuScheduler holds a weak reference to the CommandBuffer. The
   // CommandBuffer owns the GpuScheduler and holds a strong reference to it
@@ -170,10 +172,10 @@
   uint64 swap_buffers_count_;
   uint64 acknowledged_swap_buffers_count_;
   scoped_ptr<AcceleratedSurface> surface_;
+  scoped_ptr<Callback0::Type> wrapped_swap_buffers_callback_;
 #endif
 
   ScopedRunnableMethodFactory<GpuScheduler> method_factory_;
-  scoped_ptr<Callback0::Type> wrapped_swap_buffers_callback_;
   scoped_ptr<Callback0::Type> command_processed_callback_;
 };