[cc] Pass on BeginFrameAcks from CompositorEBFS through RWHVAura, DFH.

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 e7089fba80cb77c2b04994f50a5ede1b0cdfc5b7..d8826e06fc614d7e4c1f8d697939045ff1d2382a 100644
--- a/content/renderer/gpu/compositor_external_begin_frame_source.h
+++ b/content/renderer/gpu/compositor_external_begin_frame_source.h
@@ -43,9 +43,8 @@ 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,
-                      const cc::BeginFrameAck& ack) override {}
+                      const cc::BeginFrameAck& ack) override;
   bool IsThrottled() const override;
 
   // cc::ExternalBeginFrameSourceClient implementation.