cc: Track BeginMainFrame more precisely in CompositorTimingHistory.

cc/scheduler/scheduler.h

Index: cc/scheduler/scheduler.h
diff --git a/cc/scheduler/scheduler.h b/cc/scheduler/scheduler.h
index 58ad8000cc6128691198d87a916bb85d733bc768..31a19fc7f9b4390a567118e900b4d6c7f7c0f1ec 100644
--- a/cc/scheduler/scheduler.h
+++ b/cc/scheduler/scheduler.h
@@ -125,7 +125,11 @@ class CC_EXPORT Scheduler : public BeginFrameObserverBase {
     return state_machine_.impl_latency_takes_priority();
   }
 
-  void NotifyBeginMainFrameStarted();
+  // Pass in a main_thread_start_time of base::TimeTicks() if it is not
+  // known (e.g. blink lives on a remote machine) to signal that the
+  // start time isn't known and should not be used for scheduling
+  // and statistics puposes.

[Sami, 2015/11/05 14:42:36]

nit: I'm not sure if the remote machine example is more mystifying or helpful :)
Would the single thread proxy case be better?

[brianderson, 2015/11/05 21:33:26]

With STP, we can still get valid timestamps. I checked with dtrainor and he said
I could change this to "(e.g. blink is running on a remote channel)" which
aligns more with where the code for "channel_main.h" is heading.

[Sami, 2015/11/06 11:27:49]

Ah right, looks like I misread STP as passing in a zero timeticks but that's
actually not the case. Speaking about channels sgtm.

+  void NotifyBeginMainFrameStarted(base::TimeTicks main_thread_start_time);
 
   base::TimeTicks LastBeginImplFrameTime();
 
@@ -173,6 +177,7 @@ class CC_EXPORT Scheduler : public BeginFrameObserverBase {
   SchedulerStateMachine::BeginImplFrameDeadlineMode
       begin_impl_frame_deadline_mode_;
   BeginFrameTracker begin_impl_frame_tracker_;
+  BeginFrameArgs begin_main_frame_args_;
 
   base::Closure begin_retro_frame_closure_;
   base::Closure begin_impl_frame_deadline_closure_;