Delay UpdateRect until the SwapBuffers callback when accelerated compositing is on.

content/browser/renderer_host/render_message_filter.cc

Index: content/browser/renderer_host/render_message_filter.cc
diff --git a/content/browser/renderer_host/render_message_filter.cc b/content/browser/renderer_host/render_message_filter.cc
index fe937b20804296ad6e91353b0f27f424a671bcb9..238a1a27271cda7943b857aaa5a6cce40585911b 100644
--- a/content/browser/renderer_host/render_message_filter.cc
+++ b/content/browser/renderer_host/render_message_filter.cc
@@ -357,6 +357,7 @@ bool RenderMessageFilter::OnMessageReceived(const IPC::Message& message,
                         OnOpenChannelToPpapiBroker)
     IPC_MESSAGE_HANDLER_GENERIC(ViewHostMsg_UpdateRect,
         render_widget_helper_->DidReceiveUpdateMsg(message))
+    IPC_MESSAGE_HANDLER(ViewHostMsg_UpdateIsDelayed, OnUpdateIsDelayed)
     IPC_MESSAGE_HANDLER(DesktopNotificationHostMsg_CheckPermission,
                         OnCheckNotificationPermission)
     IPC_MESSAGE_HANDLER(ChildProcessHostMsg_SyncAllocateSharedMemory,
@@ -889,3 +890,16 @@ void RenderMessageFilter::OnCompletedOpenChannelToNpapiPlugin(
   DCHECK(ContainsKey(plugin_host_clients_, client));
   plugin_host_clients_.erase(client);
 }
+
+void RenderMessageFilter::OnUpdateIsDelayed(const IPC::Message& msg) {
+  // When not in accelerated compositing mode, in certain cases (e.g. waiting
+  // for a resize or if no backing store) the RenderWidgetHost is blocking the
+  // UI thread for some time, waiting for an UpdateRect from the renderer. If we
+  // are going to switch to accelerated compositing, the GPU process may need
+  // round-trips to the UI thread before finishing the frame, causing deadlocks
+  // if we delay the UpdateRect until we receive the OnSwapBuffersComplete. So
+  // the renderer sent us this message, so that we can unblock the UI thread.
+  // We will simply re-use the UpdateRect unblock mechanism, just with a
+  // different message.
+  render_widget_helper_->DidReceiveUpdateMsg(msg);
+}