mash: Remove WmRootWindowControllerObserver.

ui/display/display_observer.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/display/display_observer.h"
 
 namespace display {
 
 DisplayObserver::~DisplayObserver() {}
 
+void DisplayObserver::OnDisplayAdded(const Display& new_display) {}
+
+void DisplayObserver::OnDisplayRemoved(const Display& old_display) {}
+
+void DisplayObserver::OnDisplayMetricsChanged(const Display& display,
+                                              uint32_t changed_metrics) {}
+
 }  // namespace display