cc: Introduce BeginFrame sequence numbers and acknowledgements.

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..839aa090d8873a720ef95d1bb420752e8a383583 100644
--- a/content/renderer/gpu/compositor_external_begin_frame_source.cc
+++ b/content/renderer/gpu/compositor_external_begin_frame_source.cc
@@ -55,6 +55,12 @@ void CompositorExternalBeginFrameSource::RemoveObserver(
   external_begin_frame_source_.RemoveObserver(obs);
 }
 
+void CompositorExternalBeginFrameSource::DidFinishFrame(
+    cc::BeginFrameObserver* obs,
+    const cc::BeginFrameAck& ack) {
+  external_begin_frame_source_.DidFinishFrame(obs, ack);
+}
+
 bool CompositorExternalBeginFrameSource::IsThrottled() const {
   return true;
 }
@@ -64,6 +70,13 @@ void CompositorExternalBeginFrameSource::OnNeedsBeginFrames(
   Send(new ViewHostMsg_SetNeedsBeginFrames(routing_id_, needs_begin_frames));
 }
 
+void CompositorExternalBeginFrameSource::OnDidFinishFrame(
+    const cc::BeginFrameAck& ack) {
+  // If there were updates, SwapCompositorFrame serves as ack.
+  if (!ack.has_updates)
+    Send(new ViewHostMsg_BeginFrameDidNotDraw(routing_id_, ack));
+}
+
 void CompositorExternalBeginFrameSource::OnMessageReceived(
     const IPC::Message& message) {
   DCHECK(CalledOnValidThread());