[Mac] Maximise timer slack for background tabs

base/message_loop/message_pump.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.
 
 #ifndef BASE_MESSAGE_LOOP_MESSAGE_PUMP_H_
 #define BASE_MESSAGE_LOOP_MESSAGE_PUMP_H_
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
+#include "base/message_loop/timer_slack.h"
 #include "base/threading/non_thread_safe.h"
 
 namespace base {
 
 class TimeDelta;
 class TimeTicks;
 
 class BASE_EXPORT MessagePump : public NonThreadSafe {
  public:
   // Please see the comments above the Run method for an illustration of how
@@ skipping 107 matching lines @@
   // Schedule a DoWork callback to happen reasonably soon.  Does nothing if a
   // DoWork callback is already scheduled.  This method may be called from any
   // thread.  Once this call is made, DoWork should not be "starved" at least
   // until it returns a value of false.
   virtual void ScheduleWork() = 0;
 
   // Schedule a DoDelayedWork callback to happen at the specified time,
   // cancelling any pending DoDelayedWork callback.  This method may only be
   // used on the thread that called Run.
   virtual void ScheduleDelayedWork(const TimeTicks& delayed_work_time) = 0;
+
+  // Sets the timer slack to the specified value.
+  virtual void SetTimerSlack(TimerSlack timer_slack);
 };
 
 }  // namespace base
 
 #endif  // BASE_MESSAGE_LOOP_MESSAGE_PUMP_H_