Get rid of virtual Display::CreateScheduler.

cc/scheduler/begin_frame_source.h

Index: cc/scheduler/begin_frame_source.h
diff --git a/cc/scheduler/begin_frame_source.h b/cc/scheduler/begin_frame_source.h
index 8a3034cc702bdd4415105c9c902d31f704ad6d46..0a8a51f08d7c911d6c1594b1623ae74ad5a9fe43 100644
--- a/cc/scheduler/begin_frame_source.h
+++ b/cc/scheduler/begin_frame_source.h
@@ -114,6 +114,15 @@ class CC_EXPORT BeginFrameSource {
   virtual void RemoveObserver(BeginFrameObserver* obs) = 0;
 };
 
+// A BeginFrameSource that does nothing.
+class StubBeginFrameSource : public BeginFrameSource {
+ public:
+  void DidFinishFrame(BeginFrameObserver* obs,
+                      size_t remaining_frames) override {}
+  void AddObserver(BeginFrameObserver* obs) override {}
+  void RemoveObserver(BeginFrameObserver* obs) override {}
+};
+
 // A frame source which ticks itself independently.
 class CC_EXPORT SyntheticBeginFrameSource : public BeginFrameSource {
  public: