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 575ab06beb450555065d2801b7f388c3b4180bdc..9316cd4375df7f0ee2fc78bc5d07bd287e06302e 100644
--- a/cc/scheduler/begin_frame_source.h
+++ b/cc/scheduler/begin_frame_source.h
@@ -122,6 +122,15 @@ 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 {}
+};
+
 // Simple base class which implements a BeginFrameSource.
 // Implementation classes should:
 //  - Implement the pure virtual (Set)NeedsBeginFrames methods from