[scheduler] Add unthrottled-by-blockable task queue to fix touch latency regression.

third_party/WebKit/Source/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 "platform/wtf/RefPtr.h"
 
 #include <memory>
@@ skipping 25 matching lines @@
 
   // Set whether this frame is suspended. Only unthrottledTaskRunner tasks are
   // allowed to run on a suspended frame.
   virtual void SetSuspended(bool) {}
 
   // Set whether this frame is cross origin w.r.t. the top level frame. Cross
   // origin frames may use a different scheduling policy from same origin
   // frames.
   virtual void SetCrossOrigin(bool) {}
 
+  // The tasks runners below are listed in increasing QoS order.
+  // - timer task queue. Designed for custom user-provided javascript tasks.
+  //   Lowest guarantees. Can be suspended, blocked during user gesture or
+  //   throttled when backgrounded.
+  // - loading task queue. Can be suspended or blocked during user gesture.
+  //   Throttling might be considered in the future.
+  // - suspendable task queue. Can be suspended and blocked during user gesture,
+  //   can't be throttled.
+  // - unthrottled-but-blockable task queue. Can't be throttled, can't
+  //   be suspended but can be blocked during user gesture.
+  //   NOTE: existence of this queue is a temporary fix for scroll latency
+  //   regression. All tasks should be moved from this queue to suspendable
+  //   or unthrottled queues and it should be deleted.
+  // - unthrottled task queue. Highest guarantees. Can't be throttled,
+  //   suspended or blocked. Should be used only when necessary after
+  //   consulting scheduler-dev@.
+
+  // Returns the WebTaskRunner for timer tasks.
+  // WebFrameScheduler owns the returned WebTaskRunner.
+  virtual RefPtr<WebTaskRunner> TimerTaskRunner() = 0;
+
   // Returns the WebTaskRunner for loading tasks.
   // WebFrameScheduler owns the returned WebTaskRunner.
   virtual RefPtr<WebTaskRunner> LoadingTaskRunner() = 0;
 
-  // Returns the WebTaskRunner for timer tasks.
-  // WebFrameScheduler owns the returned WebTaskRunner.
-  virtual RefPtr<WebTaskRunner> TimerTaskRunner() = 0;
-
   // Returns the WebTaskRunner for tasks which shouldn't get throttled,
   // but can be suspended.
   // TODO(altimin): This is a transitional task runner. Unthrottled task runner
   // would become suspendable in the nearest future and a new unsuspended
   // task runner will be added.
   virtual RefPtr<WebTaskRunner> SuspendableTaskRunner() = 0;
 
+  // Retuns the WebTaskRunner for tasks which should not be suspended or
+  // throttled, but should be blocked during user gesture.
+  // This is a temporary task runner needed for a fix for touch latency
+  // regression. All tasks from it should be moved to suspendable or
+  // unthrottled task runner.
+  virtual RefPtr<WebTaskRunner> UnthrottledButBlockableTaskRunner() = 0;
+
   // Returns the WebTaskRunner for tasks which should never get throttled.
   // This is generally used for executing internal browser tasks which should
   // never be throttled. Ideally only tasks whose performance characteristics
   // are known should be posted to this task runner; for example user
   // JavaScript is discouraged. WebFrameScheduler owns the returned
   // WebTaskRunner.
   virtual RefPtr<WebTaskRunner> UnthrottledTaskRunner() = 0;
 
   // Returns the parent WebViewScheduler.
   virtual WebViewScheduler* GetWebViewScheduler() { return nullptr; }
@@ skipping 19 matching lines @@
   // connection (websocket, webrtc, etc). When connection is closed this handle
   // must be destroyed.
   virtual std::unique_ptr<ActiveConnectionHandle> OnActiveConnectionCreated() {
     return nullptr;
   };
 };
 
 }  // namespace blink
 
 #endif  // WebFrameScheduler_h