scheduler: Add an unthrottled per-frame task runner

components/scheduler/renderer/renderer_scheduler.h

Index: components/scheduler/renderer/renderer_scheduler.h
diff --git a/components/scheduler/renderer/renderer_scheduler.h b/components/scheduler/renderer/renderer_scheduler.h
index f915feb0a04dcfd45abc801ba86d5cbafaa2d073..33d75ab25b317cdfb74e206dd96a278552b61687 100644
--- a/components/scheduler/renderer/renderer_scheduler.h
+++ b/components/scheduler/renderer/renderer_scheduler.h
@@ -68,6 +68,10 @@ class SCHEDULER_EXPORT RendererScheduler : public ChildScheduler {
   // Returns a new timer task runner. This queue is intended for DOM Timers.
   virtual scoped_refptr<TaskQueue> NewTimerTaskRunner(const char* name) = 0;
 
+  // Returns a task runner for tasks which should never get throttled.

[alex clarke (OOO till 29th), 2016/07/01 11:30:05]

Not necessarily for this patch, but I wonder if we should consider adding a
builder interface here, especially since we're thinking of supporting sub-task
queues and worklets (if those get traction).

[Sami, 2016/07/01 13:11:43]

That's a fine idea -- let's keep it in mind.

+  virtual scoped_refptr<TaskQueue> NewUnthrottledTaskRunner(
+      const char* name) = 0;
+
   // Returns a new RenderWidgetSchedulingState.  The signals from this will be
   // used to make scheduling decisions.
   virtual std::unique_ptr<RenderWidgetSchedulingState>