Move DisplayChangeObserver + tests from ash to ui.

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index 0234a6cfa660328bdd5494defb31578867d90e30..bb1427219a7d9c3d2e453eb36030398b1fa85eaf 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -39,6 +39,7 @@ class AudioA11yController;
 }
 
 namespace display {
+class DisplayChangeObserver;
 class DisplayManager;
 }
 
@@ -73,7 +74,6 @@ class AcceleratorControllerDelegateAura;
 class AshNativeCursorManager;
 class AutoclickController;
 class BluetoothNotificationController;
-class DisplayChangeObserver;
 class DisplayColorManager;
 class DisplayConfigurationController;
 class DisplayErrorObserver;
@@ -509,7 +509,7 @@ class ASH_EXPORT Shell : public SystemModalContainerEventFilterDelegate,
   std::unique_ptr<ProjectingObserver> projecting_observer_;
 
   // Listens for output changes and updates the display manager.
-  std::unique_ptr<DisplayChangeObserver> display_change_observer_;
+  std::unique_ptr<display::DisplayChangeObserver> display_change_observer_;
 
   // Listens for shutdown and updates DisplayConfigurator.
   std::unique_ptr<ShutdownObserver> shutdown_observer_;