Chromium roll (https://codereview.chromium.org/976353002)

base/timer/timer.h

 // Copyright (c) 2012 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.
 
 // OneShotTimer and RepeatingTimer provide a simple timer API.  As the names
 // suggest, OneShotTimer calls you back once after a time delay expires.
 // RepeatingTimer on the other hand calls you back periodically with the
 // prescribed time interval.
 //
 // OneShotTimer and RepeatingTimer both cancel the timer when they go out of
@@ skipping 71 matching lines @@
 
   virtual ~Timer();
 
   // Returns true if the timer is running (i.e., not stopped).
   virtual bool IsRunning() const;
 
   // Returns the current delay for this timer.
   virtual TimeDelta GetCurrentDelay() const;
 
   // Set the task runner on which the task should be scheduled. This method can
-  // only be called before any tasks have been scheduled.
+  // only be called before any tasks have been scheduled. The task runner must
+  // run tasks on the same thread the timer is used on.
   virtual void SetTaskRunner(scoped_refptr<SingleThreadTaskRunner> task_runner);
 
   // Start the timer to run at the given |delay| from now. If the timer is
   // already running, it will be replaced to call the given |user_task|.
   virtual void Start(const tracked_objects::Location& posted_from,
                      TimeDelta delay,
                      const base::Closure& user_task);
 
   // Call this method to stop and cancel the timer.  It is a no-op if the timer
   // is not running.
@@ skipping 155 matching lines @@
       : Timer(posted_from, delay,
               base::Bind(method, base::Unretained(receiver)),
               false) {}
 
   void Reset() override { Timer::Reset(); }
 };
 
 }  // namespace base
 
 #endif  // BASE_TIMER_TIMER_H_