[cc] Add and use BeginFrameAck for DidFinishFrame.

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 a7ba684d934c6ca5564ce6181d00fd18768c959c..d05e2dfeb2fab0a12eb3dc1891356d2d97857608 100644
--- a/cc/scheduler/begin_frame_source.cc
+++ b/cc/scheduler/begin_frame_source.cc
@@ -100,8 +100,8 @@ void BackToBackBeginFrameSource::RemoveObserver(BeginFrameObserver* obs) {
 }
 
 void BackToBackBeginFrameSource::DidFinishFrame(BeginFrameObserver* obs,
-                                                size_t remaining_frames) {
-  if (remaining_frames == 0 && observers_.find(obs) != observers_.end()) {
+                                                const BeginFrameAck& ack) {
+  if (ack.remaining_frames == 0 && observers_.find(obs) != observers_.end()) {
     pending_begin_frame_observers_.insert(obs);
     time_source_->SetActive(true);
   }