[cc] Track observer status in ExternalBeginFrameSource.

content/renderer/gpu/compositor_external_begin_frame_source.cc

Index: content/renderer/gpu/compositor_external_begin_frame_source.cc
diff --git a/content/renderer/gpu/compositor_external_begin_frame_source.cc b/content/renderer/gpu/compositor_external_begin_frame_source.cc
index c3a8cf7d44a4c9b856b78c584f544a6453115cc1..90f5554ba3915d1deb71674f38f7eaaed7716598 100644
--- a/content/renderer/gpu/compositor_external_begin_frame_source.cc
+++ b/content/renderer/gpu/compositor_external_begin_frame_source.cc
@@ -64,6 +64,11 @@ void CompositorExternalBeginFrameSource::OnNeedsBeginFrames(
   Send(new ViewHostMsg_SetNeedsBeginFrames(routing_id_, needs_begin_frames));
 }
 
+void CompositorExternalBeginFrameSource::OnDidFinishFrame(
+    const cc::BeginFrameAck& ack) {
+  // TODO(eseckler): Pass on the ack to the view host.
+}
+
 void CompositorExternalBeginFrameSource::OnMessageReceived(
     const IPC::Message& message) {
   DCHECK(CalledOnValidThread());