cc: Introduce BeginFrame sequence numbers and acknowledgements.

cc/test/fake_external_begin_frame_source.h

Index: cc/test/fake_external_begin_frame_source.h
diff --git a/cc/test/fake_external_begin_frame_source.h b/cc/test/fake_external_begin_frame_source.h
index 8f2d4f0998e862ba9ddd489c91d77adfbce6f232..baa7b3c2c98f10125d170435ef9d51074b34e4d0 100644
--- a/cc/test/fake_external_begin_frame_source.h
+++ b/cc/test/fake_external_begin_frame_source.h
@@ -35,7 +35,7 @@ class FakeExternalBeginFrameSource
   void AddObserver(BeginFrameObserver* obs) override;
   void RemoveObserver(BeginFrameObserver* obs) override;
   void DidFinishFrame(BeginFrameObserver* obs,
-                      size_t remaining_frames) override {}
+                      const BeginFrameAck& ack) override {}
   bool IsThrottled() const override;
 
   void TestOnBeginFrame(const BeginFrameArgs& args);
@@ -49,6 +49,7 @@ class FakeExternalBeginFrameSource
   const double milliseconds_per_frame_;
   Client* client_ = nullptr;
   bool paused_ = false;
+  uint64_t next_begin_frame_number_ = 1;
   std::set<BeginFrameObserver*> observers_;
   base::CancelableCallback<void(const BeginFrameArgs&)> begin_frame_task_;
   base::WeakPtrFactory<FakeExternalBeginFrameSource> weak_ptr_factory_;