Allow null begin frame sources in cc::Scheduler

cc/scheduler/scheduler.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_SCHEDULER_H_
 #define CC_SCHEDULER_SCHEDULER_H_
 
 #include <deque>
 #include <string>
 
@@ skipping 144 matching lines @@
 
   // Virtual for testing.
   virtual base::TimeTicks Now() const;
 
   const SchedulerSettings settings_;
   // Not owned.
   SchedulerClient* client_;
   int layer_tree_host_id_;
   base::SingleThreadTaskRunner* task_runner_;
 
-  // Not owned.
+  // Not owned.  May be null.
   BeginFrameSource* begin_frame_source_;
   bool observing_begin_frame_source_;
 
   scoped_ptr<CompositorTimingHistory> compositor_timing_history_;
   base::TimeDelta estimated_parent_draw_time_;
 
   std::deque<BeginFrameArgs> begin_retro_frame_args_;
   SchedulerStateMachine::BeginImplFrameDeadlineMode
       begin_impl_frame_deadline_mode_;
   BeginFrameTracker begin_impl_frame_tracker_;
@@ skipping 44 matching lines @@
   }
 
   base::WeakPtrFactory<Scheduler> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(Scheduler);
 };
 
 }  // namespace cc
 
 #endif  // CC_SCHEDULER_SCHEDULER_H_