Allow null begin frame sources in cc::Scheduler

cc/test/scheduler_test_common.h

Index: cc/test/scheduler_test_common.h
diff --git a/cc/test/scheduler_test_common.h b/cc/test/scheduler_test_common.h
index 414f31f5d7d0161d64747bbac26eefbce826e75d..eac4983a6596283348d25fc1b0370d6b2b89d394 100644
--- a/cc/test/scheduler_test_common.h
+++ b/cc/test/scheduler_test_common.h
@@ -238,7 +238,9 @@ class TestScheduler : public Scheduler {
     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;