Added GPU process "echo" IPC message.

content/renderer/pepper_platform_context_3d_impl.h

Index: content/renderer/pepper_platform_context_3d_impl.h
===================================================================
--- content/renderer/pepper_platform_context_3d_impl.h	(revision 98655)
+++ content/renderer/pepper_platform_context_3d_impl.h	(working copy)
@@ -21,6 +21,7 @@
 class CommandBufferProxy;
 class GpuChannelHost;
 class RendererGLContext;
+class Task;
 
 class PlatformContext3DImpl
     : public webkit::ppapi::PluginDelegate::PlatformContext3D {
@@ -29,11 +30,11 @@
   virtual ~PlatformContext3DImpl();
 
   virtual bool Init(const int32* attrib_list);
-  virtual void SetSwapBuffersCallback(Callback0::Type* callback);
   virtual unsigned GetBackingTextureId();
   virtual gpu::CommandBuffer* GetCommandBuffer();
   virtual int GetCommandBufferRouteId();
   virtual void SetContextLostCallback(Callback0::Type* callback);
+  virtual bool Echo(Task* task);
 
  private:
   bool InitRaw();