Revert of Remove the concept of a cleanup task

public/platform/WebScheduler.h

 // Copyright 2014 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 WebScheduler_h
 #define WebScheduler_h
 
 #include "WebCommon.h"
 #include "public/platform/WebThread.h"
 
 namespace blink {
 
 class WebTraceLocation;
 
 // This class is used to submit tasks and pass other information from Blink to
 // the platform's scheduler.
 class BLINK_PLATFORM_EXPORT WebScheduler {
 public:
     virtual ~WebScheduler() { }
 
-    // Called to prevent any more pending tasks from running. Must be called on
-    // the associated WebThread.
-    virtual void shutdown() { }
-
     // Returns true if there is high priority work pending on the associated WebThread
     // and the caller should yield to let the scheduler service that work.
     // Must be called on the associated WebThread.
     virtual bool shouldYieldForHighPriorityWork() { return false; }
 
     // Returns true if a currently running idle task could exceed its deadline
     // without impacting user experience too much. This should only be used if
     // there is a task which cannot be pre-empted and is likely to take longer
     // than the largest expected idle task deadline. It should NOT be polled to
     // check whether more work can be performed on the current idle task after
@@ skipping 41 matching lines @@
     void postIdleTask(const WebTraceLocation&, PassOwnPtr<IdleTask>);
     void postNonNestableIdleTask(const WebTraceLocation&, PassOwnPtr<IdleTask>);
     void postIdleTaskAfterWakeup(const WebTraceLocation&, PassOwnPtr<IdleTask>);
     void postLoadingTask(const WebTraceLocation&, PassOwnPtr<Task>);
 #endif
 };
 
 } // namespace blink
 
 #endif // WebScheduler_h