scheduler: Add an unthrottled per-frame task runner

third_party/WebKit/public/platform/WebFrameScheduler.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef WebFrameScheduler_h
 #define WebFrameScheduler_h
 
 #include "WebCommon.h"
 
 #include <string>
@@ skipping 14 matching lines @@
     // The scheduler may throttle tasks associated with pages that are not visible.
     virtual void setPageVisible(bool) { }
 
     // Returns the WebTaskRunner for loading tasks.
     // WebFrameScheduler owns the returned WebTaskRunner.
     virtual WebTaskRunner* loadingTaskRunner() { return nullptr; }
 
     // 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
 
 #endif // WebFrameScheduler_h