Hook up ui::Compositor to Display's BeginFrameSource

cc/surfaces/display_scheduler.cc

Index: cc/surfaces/display_scheduler.cc
diff --git a/cc/surfaces/display_scheduler.cc b/cc/surfaces/display_scheduler.cc
index 28f780862bc10c93388c09b76d6c45f93556fc51..9fb863cb1f8e527b491130dfe543170b5b4e3019 100644
--- a/cc/surfaces/display_scheduler.cc
+++ b/cc/surfaces/display_scheduler.cc
@@ -34,9 +34,12 @@ DisplayScheduler::DisplayScheduler(DisplaySchedulerClient* client,
   begin_frame_deadline_closure_ = base::Bind(
       &DisplayScheduler::OnBeginFrameDeadline, weak_ptr_factory_.GetWeakPtr());
 
-  // TODO(tansell): Set this to something useful.
+  // TODO(enne): remove this once the renderer uses begin frames everywhere.
   begin_frame_source_for_children_ = SyntheticBeginFrameSource::Create(
       task_runner, BeginFrameArgs::DefaultInterval());
+
+  // TODO(enne): For now, this is always the same, so just set initially.
+  client_->UpdateSchedulerBeginFrameSource(begin_frame_source_);

[enne (OOO), 2016/03/25 17:55:36]

I'm not sure about the split between DisplayScheduler and Display in terms of
registering BeginFrameSources.  Right now I left it as the DisplayScheduler
picking the top level BFS it wants to use (which could be unthrottled, and isn't
in this patch yet), and then the Display registering it with the current surface
id and handling surface id changes (which I don't think happen, but that's not
asserted anywhere).

 }
 
 DisplayScheduler::~DisplayScheduler() {