Use a mocked compositor for unit tests

ui/aura/desktop.h

Index: ui/aura/desktop.h
diff --git a/ui/aura/desktop.h b/ui/aura/desktop.h
index debf1d7007a6298d7a524e3388c0b777c9946b28..cc4cd10c120cf5f00b2e46c384239d52b598d0f9 100644
--- a/ui/aura/desktop.h
+++ b/ui/aura/desktop.h
@@ -49,13 +49,6 @@ class AURA_EXPORT Desktop : public ui::CompositorDelegate,
   static Desktop* GetInstance();
   static void DeleteInstanceForTesting();
 
-  static void set_compositor_factory_for_testing(ui::Compositor*(*factory)()) {
-    compositor_factory_ = factory;
-  }
-  static ui::Compositor* (*compositor_factory())() {
-    return compositor_factory_;
-  }
-
   ui::Compositor* compositor() { return compositor_.get(); }
   gfx::Point last_mouse_location() const { return last_mouse_location_; }
   DesktopDelegate* delegate() { return delegate_.get(); }
@@ -170,9 +163,6 @@ class AURA_EXPORT Desktop : public ui::CompositorDelegate,
   // Used to schedule painting.
   base::WeakPtrFactory<Desktop> schedule_paint_factory_;
 
-  // Factory used to create Compositors. Settable by tests.
-  static ui::Compositor*(*compositor_factory_)();
-
   Window* active_window_;
 
   // Last location seen in a mouse event.