[cc] Add and use BeginFrameAck for DidFinishFrame.

content/renderer/gpu/compositor_external_begin_frame_source.h

Index: content/renderer/gpu/compositor_external_begin_frame_source.h
diff --git a/content/renderer/gpu/compositor_external_begin_frame_source.h b/content/renderer/gpu/compositor_external_begin_frame_source.h
index 4b41f2a136f08d5e69762e599d29d73b151f457e..7075023e95a2bb17d4e6e91c84ae64b391572938 100644
--- a/content/renderer/gpu/compositor_external_begin_frame_source.h
+++ b/content/renderer/gpu/compositor_external_begin_frame_source.h
@@ -42,8 +42,9 @@ class CompositorExternalBeginFrameSource
   // cc::BeginFrameSource implementation.
   void AddObserver(cc::BeginFrameObserver* obs) override;
   void RemoveObserver(cc::BeginFrameObserver* obs) override;
+  // TODO(eseckler): Track and forward BeginFrameAcks to browser.
   void DidFinishFrame(cc::BeginFrameObserver* obs,
-                      size_t remaining_frames) override {}
+                      const cc::BeginFrameAck& ack) override {}
   bool IsThrottled() const override;
 
   // cc::ExternalBeginFrameSourceClient implementation.