Revert of gpu: Completely remove GpuProcessHostUIShim.

content/browser/gpu/gpu_process_host.h

Index: content/browser/gpu/gpu_process_host.h
diff --git a/content/browser/gpu/gpu_process_host.h b/content/browser/gpu/gpu_process_host.h
index bd98b70c0212e387873314d736f00e191ea89f33..7b25454b7e93a7f81bccf3933fd9914ca903f770 100644
--- a/content/browser/gpu/gpu_process_host.h
+++ b/content/browser/gpu/gpu_process_host.h
@@ -173,6 +173,9 @@
 
   bool Init();
 
+  // Post an IPC message to the UI shim's message handler on the UI thread.
+  void RouteOnUIThread(const IPC::Message& message);
+
   // BrowserChildProcessHostDelegate implementation.
   bool OnMessageReceived(const IPC::Message& message) override;
   void OnChannelConnected(int32_t peer_pid) override;