scheduler: Add an unthrottled per-frame task runner

third_party/WebKit/public/platform/WebFrameScheduler.h

Index: third_party/WebKit/public/platform/WebFrameScheduler.h
diff --git a/third_party/WebKit/public/platform/WebFrameScheduler.h b/third_party/WebKit/public/platform/WebFrameScheduler.h
index e1454b3b71cc5dac8317340bdff3aa996d4bfeb6..16a06f2db79a958ca1c366e2f100a67d90539f44 100644
--- a/third_party/WebKit/public/platform/WebFrameScheduler.h
+++ b/third_party/WebKit/public/platform/WebFrameScheduler.h
@@ -32,6 +32,10 @@ public:
     // Returns the WebTaskRunner for timer tasks.
     // WebFrameScheduler owns the returned WebTaskRunner.
     virtual WebTaskRunner* timerTaskRunner() { return nullptr; }
+
+    // Returns the WebTaskRunner for tasks which should never get throttled.
+    // WebFrameScheduler owns the returned WebTaskRunner.

[dcheng, 2016/07/04 06:52:35]

Will we have some sort of documentation for what tasks go in which queue? For
example, this CL changes one line in FrameView to use the unthrottled task
runner, but I'm not sure why =)

[alex clarke (OOO till 29th), 2016/07/04 09:02:19]

Scheduling decisions (i.e. should a frame be throttled) are based on the
notification delivered by ThrottlingObserverNotificationFactory.  This being
posted on a potentially throttled queue is undesirable since it could take up to
a second for task queues associated with the frame to become unthrottled when it
becomes visible.

Posting on the unthrottled queue should make it happen almost immediately.

[Sami, 2016/07/04 09:28:43]

I've added a bit more documentation here to hopefully make the intended usage
clearer. 

+    virtual WebTaskRunner* unthrottledTaskRunner() { return nullptr; }
 };
 
 } // namespace blink