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

cc/test/fake_external_begin_frame_source.cc

Index: cc/test/fake_external_begin_frame_source.cc
diff --git a/cc/test/fake_external_begin_frame_source.cc b/cc/test/fake_external_begin_frame_source.cc
index 035eb89e72cc63d1c7bf01d76bfac15971109b2b..8398dcfc16c07f4f814513db2fc5cf63319bb67f 100644
--- a/cc/test/fake_external_begin_frame_source.cc
+++ b/cc/test/fake_external_begin_frame_source.cc
@@ -66,10 +66,7 @@ void FakeExternalBeginFrameSource::RemoveObserver(BeginFrameObserver* obs) {
     client_->OnRemoveObserver(obs);
 }
 
-void FakeExternalBeginFrameSource::DidFinishFrame(BeginFrameObserver* obs,
-                                                  const BeginFrameAck& ack) {
-  last_acks_[obs] = ack;
-}
+void FakeExternalBeginFrameSource::DidFinishFrame(BeginFrameObserver* obs) {}
 
 bool FakeExternalBeginFrameSource::IsThrottled() const {
   return true;