[cc] Add and use BeginFrameAck for DidFinishFrame.

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 ced3b3ba4fab29d7914a32f703304cbc116976c4..d02fe0bc7fb11b0b30fe716b5c2f88962aaba4ff 100644
--- a/cc/test/fake_external_begin_frame_source.h
+++ b/cc/test/fake_external_begin_frame_source.h
@@ -36,7 +36,7 @@ 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);
@@ -50,6 +50,7 @@ class FakeExternalBeginFrameSource
   const double milliseconds_per_frame_;
   Client* client_ = nullptr;
   bool paused_ = false;
+  BeginFrameArgs current_args_;
   uint64_t next_begin_frame_number_ = BeginFrameArgs::kStartingFrameNumber;
   std::set<BeginFrameObserver*> observers_;
   base::CancelableCallback<void(const BeginFrameArgs&)> begin_frame_task_;