Added GPU process "echo" IPC message.

content/common/gpu/gpu_channel.cc

Index: content/common/gpu/gpu_channel.cc
===================================================================
--- content/common/gpu/gpu_channel.cc	(revision 98655)
+++ content/common/gpu/gpu_channel.cc	(working copy)
@@ -260,6 +260,7 @@
     IPC_MESSAGE_HANDLER(GpuChannelMsg_DestroySurface, OnDestroySurface)
     IPC_MESSAGE_HANDLER(GpuChannelMsg_CreateTransportTexture,
         OnCreateTransportTexture)
+    IPC_MESSAGE_HANDLER(GpuChannelMsg_Echo, OnEcho);
     IPC_MESSAGE_UNHANDLED(handled = false)
   IPC_END_MESSAGE_MAP()
   DCHECK(handled) << msg.type();
@@ -411,6 +412,11 @@
  #endif
 }
 
+void GpuChannel::OnEcho(const IPC::Message& message) {
+  TRACE_EVENT0("gpu", "GpuCommandBufferStub::OnEcho");
+  Send(new IPC::Message(message));
+}
+
 bool GpuChannel::Init(base::MessageLoopProxy* io_message_loop,
                       base::WaitableEvent* shutdown_event) {
   // Check whether we're already initialized.