Allow null begin frame sources in cc::Scheduler

cc/test/scheduler_test_common.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_TEST_SCHEDULER_TEST_COMMON_H_
 #define CC_TEST_SCHEDULER_TEST_COMMON_H_
 
 #include <stddef.h>
 
 #include <string>
@@ skipping 220 matching lines @@
   bool NeedsBeginMainFrame() const {
     return state_machine_.needs_begin_main_frame();
   }
 
   BeginFrameSource& frame_source() { return *begin_frame_source_; }
 
   bool MainThreadMissedLastDeadline() const {
     return state_machine_.main_thread_missed_last_deadline();
   }
 
-  bool begin_frames_expected() const { return observing_begin_frame_source_; }
+  bool begin_frames_expected() const {
+    return begin_frame_source_ && observing_begin_frame_source_;
+  }
 
   ~TestScheduler() override;
 
   base::TimeDelta BeginImplFrameInterval() {
     return begin_impl_frame_tracker_.Interval();
   }
 
   // Note: This setting will be overriden on the next BeginFrame in the
   // scheduler. To control the value it gets on the next BeginFrame
   // Pass in a fake CompositorTimingHistory that indicates BeginMainFrame
   // to Activation is fast.
   void SetCriticalBeginMainFrameToActivateIsFast(bool is_fast) {
     state_machine_.SetCriticalBeginMainFrameToActivateIsFast(is_fast);
   }
 
  protected:
   // Overridden from Scheduler.
   base::TimeTicks Now() const override;
 
  private:
   base::SimpleTestTickClock* now_src_;
 
   DISALLOW_COPY_AND_ASSIGN(TestScheduler);
 };
 
 }  // namespace cc
 
 #endif  // CC_TEST_SCHEDULER_TEST_COMMON_H_