[scheduler]: Avoid waking up the scheduler to end long idle periods.

components/scheduler/child/nestable_single_thread_task_runner.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 COMPONENTS_SCHEDULER_CHILD_NESTABLE_SINGLE_THREAD_TASK_RUNNER_H_
 #define COMPONENTS_SCHEDULER_CHILD_NESTABLE_SINGLE_THREAD_TASK_RUNNER_H_
 
 #include "base/single_thread_task_runner.h"
+#include "base/message_loop/message_loop.h"
 #include "components/scheduler/scheduler_export.h"
 
 namespace scheduler {
 
 // A single thread task runner which exposes whether it is running nested.
 class SCHEDULER_EXPORT NestableSingleThreadTaskRunner
     : public base::SingleThreadTaskRunner {
  public:
   NestableSingleThreadTaskRunner() {}
 
   // Returns true if the task runner is nested (i.e., running a run loop within
   // a nested task).
   virtual bool IsNested() const = 0;
 
+  // Adds and removes MessageLoop::TaskObservers to the task runner.
+  virtual void AddTaskObserver(
+      base::MessageLoop::TaskObserver* task_observer) = 0;
+  virtual void RemoveTaskObserver(
+      base::MessageLoop::TaskObserver* task_observer) = 0;
+
  protected:
   ~NestableSingleThreadTaskRunner() override {}
 
   DISALLOW_COPY_AND_ASSIGN(NestableSingleThreadTaskRunner);
 };
 
 }  // namespace scheduler
 
 #endif  // COMPONENTS_SCHEDULER_CHILD_NESTABLE_SINGLE_THREAD_TASK_RUNNER_H_