[cc] Remove BeginFrameAck from BFS::DidFinishFrame and update tests.

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 af0c0a99274e671ce0259a012c5447b9b1719f74..d98de84c0400674521d727c30c91681f86bca22f 100644
--- a/cc/test/fake_external_begin_frame_source.h
+++ b/cc/test/fake_external_begin_frame_source.h
@@ -39,8 +39,7 @@ class FakeExternalBeginFrameSource
   // BeginFrameSource implementation.
   void AddObserver(BeginFrameObserver* obs) override;
   void RemoveObserver(BeginFrameObserver* obs) override;
-  void DidFinishFrame(BeginFrameObserver* obs,
-                      const BeginFrameAck& ack) override;
+  void DidFinishFrame(BeginFrameObserver* obs) override;
   bool IsThrottled() const override;
 
   BeginFrameArgs CreateBeginFrameArgs(
@@ -53,10 +52,6 @@ class FakeExternalBeginFrameSource
 
   size_t num_observers() const { return observers_.size(); }
 
-  const BeginFrameAck& LastAckForObserver(BeginFrameObserver* obs) {
-    return last_acks_[obs];
-  }
-
  private:
   void PostTestOnBeginFrame();