Don't create mirror window during bootstrap

ash/display/display_controller.h

Index: ash/display/display_controller.h
diff --git a/ash/display/display_controller.h b/ash/display/display_controller.h
index 948a7ad00618389c5716c940c7d412741631d5a4..e3311141d1b81f5537b2d2ecc2859c84d2685bfe 100644
--- a/ash/display/display_controller.h
+++ b/ash/display/display_controller.h
@@ -38,6 +38,7 @@ namespace ash {
 namespace internal {
 class DisplayManager;
 class FocusActivationStore;
+class MirrorWindowController;
 class RootWindowController;
 }
 
@@ -166,6 +167,7 @@ class ASH_EXPORT DisplayController : public gfx::DisplayObserver {
 
  private:
   friend class internal::DisplayManager;
+  friend class internal::MirrorWindowController;
 
   // Creates a root window for |display| and stores it in the |root_windows_|
   // map.