GetTimeToCallback() returns estimated time to callback.

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 82 matching lines @@
         TickClock* tick_clock);
 
   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;
 
+  // Returns an estimated time to the timer calling the user_task_ back.
+  // Note that the time may be negative if the timer has not been started,
+  // is late to call back or has done its last call back.
+  virtual TimeDelta GetTimeToCallback() const;
+
   // Set the task runner on which the task should be scheduled. This method can
   // 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);
@@ skipping 188 matching lines @@
 // to link in MSVC. But clang-plugin does not allow inline definitions of
 // virtual methods, so the inline definition lives in the header file here
 // to satisfy both.
 inline void DelayTimer::Reset() {
   Timer::Reset();
 }
 
 }  // namespace base
 
 #endif  // BASE_TIMER_TIMER_H_