Refactoring the way begin frame sources inside scheduler work.

cc/scheduler/time_source.h

 // Copyright 2011 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 CC_SCHEDULER_TIME_SOURCE_H_
 #define CC_SCHEDULER_TIME_SOURCE_H_
 
 #include "base/memory/ref_counted.h"
 #include "base/time/time.h"
 #include "cc/base/cc_export.h"
@@ skipping 18 matching lines @@
   virtual void SetClient(TimeSourceClient* client) = 0;
 
   // If transitioning from not active to active, SetActive will return the
   // timestamp of the most recenly missed tick that did not have OnTimerTick
   // called.
   virtual base::TimeTicks SetActive(bool active) = 0;
 
   virtual bool Active() const = 0;
   virtual void SetTimebaseAndInterval(base::TimeTicks timebase,
                                       base::TimeDelta interval) = 0;
-  virtual base::TimeTicks LastTickTime() = 0;
-  virtual base::TimeTicks NextTickTime() = 0;
+  virtual base::TimeTicks LastTickTime() const = 0;
+  virtual base::TimeTicks NextTickTime() const = 0;
 
  protected:
   virtual ~TimeSource() {}
 
  private:
   friend class base::RefCounted<TimeSource>;
 };
 
 }  // namespace cc
 
 #endif  // CC_SCHEDULER_TIME_SOURCE_H_