Break dependency between ash::DisplayManager and ash::ScreenAsh

ash/display/screen_ash.h

Index: ash/display/screen_ash.h
diff --git a/ash/display/screen_ash.h b/ash/display/screen_ash.h
index 93a26d9003ee7cf647ce303916be5a46571e1639..15337a2d988933d709b5398e1b7104c46f06e8bb 100644
--- a/ash/display/screen_ash.h
+++ b/ash/display/screen_ash.h
@@ -8,20 +8,19 @@
 #include <stdint.h>
 
 #include "ash/ash_export.h"
-#include "base/compiler_specific.h"
 #include "base/macros.h"
-#include "base/observer_list.h"
-#include "ui/display/display_observer.h"
 #include "ui/display/screen.h"
 
+namespace display {
+class DisplayObserver;
+}
+
 namespace gfx {
 class Rect;
 }
 
 namespace ash {
-namespace internal {
 class DisplayManager;
-}
 
 // Aura implementation of display::Screen. Implemented here to avoid circular
 // dependencies.
@@ -45,20 +44,13 @@ class ASH_EXPORT ScreenAsh : public display::Screen {
   void AddObserver(display::DisplayObserver* observer) override;
   void RemoveObserver(display::DisplayObserver* observer) override;
 
- private:
-  friend class DisplayManager;
+  // CreateDisplayManager with a ScreenAsh instance.
+  static DisplayManager* CreateDisplayManager();
 
-  // Notifies observers of display configuration changes.
-  void NotifyMetricsChanged(const display::Display& display, uint32_t metrics);
-  void NotifyDisplayAdded(const display::Display& display);
-  void NotifyDisplayRemoved(const display::Display& display);
-
-  // Creates a screen that can be used during shutdown.
-  // It simply has a copy of the displays.
-  display::Screen* CloneForShutdown();
-
-  base::ObserverList<display::DisplayObserver> observers_;
+  // Create a screen instance to be used during shutdown.
+  static void CreateScreenForShutdown();
 
+ private:
   DISALLOW_COPY_AND_ASSIGN(ScreenAsh);
 };