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

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 30916948cec360b751b158e69316c4e6617f8b3b..e6be12ad393628170aecd0e0030035107234d92d 100644
--- a/cc/scheduler/begin_frame_source.cc
+++ b/cc/scheduler/begin_frame_source.cc
@@ -77,7 +77,7 @@ void BackToBackBeginFrameSource::RemoveObserver(BeginFrameObserver* 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);
 }