Save display preference when all display configuration changes has been completed.

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 eb6f487577fe510e5a3435ef0aac6a2b143e9827..f58dc12162ab52a3eca08459cb4c8db543c27f05 100644
--- a/ash/display/display_controller.h
+++ b/ash/display/display_controller.h
@@ -30,6 +30,7 @@ template <typename T> class JSONValueConverter;
 
 namespace ash {
 namespace internal {
+class DisplayManager;
 class RootWindowController;
 }
 
@@ -79,6 +80,10 @@ class ASH_EXPORT DisplayController : public gfx::DisplayObserver {
     // but before the change is applied to aura/ash.
     virtual void OnDisplayConfigurationChanging() = 0;
 
+    // Invoked when the all display configuration changes
+    // have been applied.
+    virtual void OnDisplayConfigurationChanged() {};
+
    protected:
     virtual ~Observer() {}
   };
@@ -180,6 +185,8 @@ class ASH_EXPORT DisplayController : public gfx::DisplayObserver {
   virtual void OnDisplayRemoved(const gfx::Display& display) OVERRIDE;
 
  private:
+  friend class internal::DisplayManager;
+
   // Create a root window for given |display|.
   aura::RootWindow* CreateRootWindowForDisplay(const gfx::Display& display);
 
@@ -190,6 +197,7 @@ class ASH_EXPORT DisplayController : public gfx::DisplayObserver {
   void UpdateDisplayBoundsForLayout();
 
   void NotifyDisplayConfigurationChanging();
+  void NotifyDisplayConfigurationChanged();
 
   void SetLayoutForDisplayIdPair(const DisplayIdPair& display_pair,
                                  const DisplayLayout& layout);