[cc] BackToBackBFS: Disable time source if all pending observers removed

cc/scheduler/begin_frame_source.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/scheduler/begin_frame_source.h"
 
 #include <stddef.h>
 
 #include "base/auto_reset.h"
 #include "base/location.h"
@@ skipping 59 matching lines @@
   pending_begin_frame_observers_.insert(obs);
   obs->OnBeginFrameSourcePausedChanged(false);
   time_source_->SetActive(true);
 }
 
 void BackToBackBeginFrameSource::RemoveObserver(BeginFrameObserver* obs) {
   DCHECK(obs);
   DCHECK(observers_.find(obs) != observers_.end());
   observers_.erase(obs);
   pending_begin_frame_observers_.erase(obs);
-  if (observers_.empty())
+  if (pending_begin_frame_observers_.empty())
     time_source_->SetActive(false);
 }
 
 void BackToBackBeginFrameSource::DidFinishFrame(BeginFrameObserver* obs,
                                                 size_t remaining_frames) {
   if (remaining_frames == 0 && observers_.find(obs) != observers_.end()) {
     pending_begin_frame_observers_.insert(obs);
     time_source_->SetActive(true);
   }
 }
 
 bool BackToBackBeginFrameSource::IsThrottled() const {
   return false;
 }
 
 void BackToBackBeginFrameSource::OnTimerTick() {

[brianderson, 2016/12/20 16:37:36]

Would it make sense to DCHECK here that pending_begin_frame_observers_ is empty?

[Eric Seckler, 2016/12/20 16:43:33]

I assume you mean "not empty" :) Added a DCHECK below.

   base::TimeTicks frame_time = time_source_->LastTickTime();
   base::TimeDelta default_interval = BeginFrameArgs::DefaultInterval();
   BeginFrameArgs args = BeginFrameArgs::Create(
       BEGINFRAME_FROM_HERE, frame_time, frame_time + default_interval,
       default_interval, BeginFrameArgs::NORMAL);
 
   // This must happen after getting the LastTickTime() from the time source.
   time_source_->SetActive(false);
 
   std::unordered_set<BeginFrameObserver*> pending_observers;
@@ skipping 137 matching lines @@
 
 void ExternalBeginFrameSource::OnBeginFrame(const BeginFrameArgs& args) {
   missed_begin_frame_args_ = args;
   missed_begin_frame_args_.type = BeginFrameArgs::MISSED;
   std::unordered_set<BeginFrameObserver*> observers(observers_);
   for (auto* obs : observers)
     obs->OnBeginFrame(args);
 }
 
 }  // namespace cc