[cc] Track observer status in ExternalBeginFrameSource.

components/exo/compositor_frame_sink_holder.cc

Index: components/exo/compositor_frame_sink_holder.cc
diff --git a/components/exo/compositor_frame_sink_holder.cc b/components/exo/compositor_frame_sink_holder.cc
index 7d5d5f7ecc486265512b39cb226addd3bd3470fb..0a67f8cf5676f99ed0c2ca6dbb074e41eef1aeef 100644
--- a/components/exo/compositor_frame_sink_holder.cc
+++ b/components/exo/compositor_frame_sink_holder.cc
@@ -58,6 +58,7 @@ void CompositorFrameSinkHolder::DidReceiveCompositorFrameAck() {
 }
 
 void CompositorFrameSinkHolder::OnBeginFrame(const cc::BeginFrameArgs& args) {
+  // TODO(eseckler): Hook up exo::Surface to the ExternalBFS.
   if (surface_)
     surface_->BeginFrame(args.frame_time);
 
@@ -100,6 +101,10 @@ void CompositorFrameSinkHolder::OnNeedsBeginFrames(bool needs_begin_frames) {
   frame_sink_->SetNeedsBeginFrame(needs_begin_frames);
 }
 
+void CompositorFrameSinkHolder::OnDidFinishFrame(const cc::BeginFrameAck& ack) {
+  // TODO(eseckler): Pass on the ack to frame_sink_.
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // SurfaceObserver overrides: