Hook up ui::Compositor to Display's BeginFrameSource

cc/trees/single_thread_proxy.cc

Index: cc/trees/single_thread_proxy.cc
diff --git a/cc/trees/single_thread_proxy.cc b/cc/trees/single_thread_proxy.cc
index fa193bf3e0a63f7b0b0c23d1279dfaa77b61e21e..e8d515c0afe34041d72f8b0ab4d36bb8793779e2 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -489,6 +489,14 @@ void SingleThreadProxy::CommitVSyncParameters(base::TimeTicks timebase,
     synthetic_begin_frame_source_->OnUpdateVSyncParameters(timebase, interval);
 }
 
+void SingleThreadProxy::SetBeginFrameSource(BeginFrameSource* source) {
+  // This should only happen when using the scheduler.
+  DCHECK(layer_tree_host_->settings().single_thread_proxy_scheduler);
+  // TODO(enne): this overrides any preexisting begin frame source.  Those

[no sievers, 2016/03/25 18:55:08]

Yea see other comment wrt Android. Since it currently uses
|external_begin_frame_source| there, one would probably override the other. But
it's probably straightforward to just have them both use the same source now.

+  // other sources will eventually be removed and this will be the only path.
+  scheduler_on_impl_thread_->SetBeginFrameSource(source);
+}
+
 void SingleThreadProxy::SetEstimatedParentDrawTime(base::TimeDelta draw_time) {
   if (scheduler_on_impl_thread_)
     scheduler_on_impl_thread_->SetEstimatedParentDrawTime(draw_time);