[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..4b7fe01e710ca9edfd6c2711f5a8a8872b4b13e1 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);
@@ -311,9 +310,6 @@ void ExternalBeginFrameSource::RemoveObserver(BeginFrameObserver* obs) {
   }
 }
 
-void ExternalBeginFrameSource::DidFinishFrame(BeginFrameObserver* obs,
-                                              const BeginFrameAck& ack) {}
-
 bool ExternalBeginFrameSource::IsThrottled() const {
   return true;
 }