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 5b729b4e6102f386751c3e32f36040608fa2d865..6a871553b6f6d57dee778bb83c1a93168715e336 100644
--- a/ui/aura/desktop.h
+++ b/ui/aura/desktop.h
@@ -79,13 +79,6 @@ class AURA_EXPORT Desktop : public ui::CompositorDelegate {
 
   Window* window() { return window_.get(); }
 
-  static void set_compositor_factory_for_testing(ui::Compositor*(*factory)()) {
-    compositor_factory_ = factory;
-  }
-  static ui::Compositor* (*compositor_factory())() {
-    return compositor_factory_;
-  }
-
   // Sets the active window to |window| and the focused window to |to_focus|.
   // If |to_focus| is NULL, |window| is focused.
   void SetActiveWindow(Window* window, Window* to_focus);
@@ -132,9 +125,6 @@ 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.