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

cc/scheduler/begin_frame_source.cc

Index: cc/scheduler/begin_frame_source.cc
diff --git a/cc/scheduler/begin_frame_source.cc b/cc/scheduler/begin_frame_source.cc
index be6394e617a88882f5806432ef0d3de16927d2cc..1daef73e1fce96424623f1f94e0934b1cc5d1400 100644
--- a/cc/scheduler/begin_frame_source.cc
+++ b/cc/scheduler/begin_frame_source.cc
@@ -113,8 +113,7 @@ void BackToBackBeginFrameSource::RemoveObserver(BeginFrameObserver* obs) {
     time_source_->SetActive(false);
 }
 
-void BackToBackBeginFrameSource::DidFinishFrame(BeginFrameObserver* obs,
-                                                const BeginFrameAck& ack) {
+void BackToBackBeginFrameSource::DidFinishFrame(BeginFrameObserver* obs) {
   if (observers_.find(obs) != observers_.end()) {
     pending_begin_frame_observers_.insert(obs);
     time_source_->SetActive(true);
@@ -311,8 +310,7 @@ void ExternalBeginFrameSource::RemoveObserver(BeginFrameObserver* obs) {
   }
 }
 
-void ExternalBeginFrameSource::DidFinishFrame(BeginFrameObserver* obs,
-                                              const BeginFrameAck& ack) {}
+void ExternalBeginFrameSource::DidFinishFrame(BeginFrameObserver* obs) {}
 
 bool ExternalBeginFrameSource::IsThrottled() const {
   return true;