[cc] Track observer status in ExternalBeginFrameSource.

cc/surfaces/direct_compositor_frame_sink.cc

Index: cc/surfaces/direct_compositor_frame_sink.cc
diff --git a/cc/surfaces/direct_compositor_frame_sink.cc b/cc/surfaces/direct_compositor_frame_sink.cc
index 01cb8b1ee371af8f7d471aa69a2e54bd4e0f87b6..cbfc1f5d4070529c4b4c4600e3a940c70b886daf 100644
--- a/cc/surfaces/direct_compositor_frame_sink.cc
+++ b/cc/surfaces/direct_compositor_frame_sink.cc
@@ -150,4 +150,8 @@ void DirectCompositorFrameSink::OnNeedsBeginFrames(bool needs_begin_frame) {
   support_->SetNeedsBeginFrame(needs_begin_frame);
 }
 
+void DirectCompositorFrameSink::OnDidFinishFrame(const BeginFrameAck& ack) {
+  // TODO(eseckler): Pass on the ack to support_ and its BeginFrameSource.
+}
+
 }  // namespace cc