Hoist begin frame sources out of scheduler

cc/trees/single_thread_proxy.h

Index: cc/trees/single_thread_proxy.h
diff --git a/cc/trees/single_thread_proxy.h b/cc/trees/single_thread_proxy.h
index 3b69668f69969d7b7101a3fbde0a77f599b4cad0..9d5674aaf4dda9c9e9f2f90634e144980ec7e6bd 100644
--- a/cc/trees/single_thread_proxy.h
+++ b/cc/trees/single_thread_proxy.h
@@ -144,9 +144,13 @@ class CC_EXPORT SingleThreadProxy : public Proxy,
   RendererCapabilities renderer_capabilities_for_main_thread_;
 
   // Accessed from both threads.
-  scoped_ptr<BeginFrameSource> external_begin_frame_source_;
+  scoped_ptr<BeginFrameSource> begin_frame_source_;

[Sami, 2016/03/08 19:15:25]

Sorry to latch onto the naming but I would keep this as
external_begin_frame_source_. Otherwise it's not really obvious when this would
be set vs. the synthetic one. WDYT? (same comment for proxy_impl.h)

[enne (OOO), 2016/03/08 21:38:02]

This ends up being either the external one or the throttled one.  Would you
rather I have three better named variables, only one of which is valid?

[Sami, 2016/03/09 14:36:23]

The new design with precise names for all the three alternatives looks pretty
clear to me.

+  scoped_ptr<SyntheticBeginFrameSource> synthetic_frame_source_;

[Sami, 2016/03/08 19:15:25]

nit: synthetic_begin_frame_source_

   scoped_ptr<Scheduler> scheduler_on_impl_thread_;
 
+  base::TimeDelta authoritative_vsync_interval_;
+  base::TimeTicks last_vsync_timebase_;
+
   scoped_ptr<BlockingTaskRunner::CapturePostTasks> commit_blocking_task_runner_;
   bool next_frame_is_newly_committed_frame_;