Make aura work with mus

ui/aura/window_observer.cc

 // Copyright 2014 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/aura/window_observer.h"
 
 #include "base/logging.h"
 #include "ui/aura/window.h"
 
 namespace aura {
 
 WindowObserver::WindowObserver() : observing_(0) {
 }
 
 WindowObserver::~WindowObserver() {
   CHECK_EQ(0, observing_);
 }
 
 void WindowObserver::OnObservingWindow(aura::Window* window) {
   if (!window->HasObserver(this))
     observing_++;
 }
 
 void WindowObserver::OnUnobservingWindow(aura::Window* window) {
   if (window->HasObserver(this))
     observing_--;
 }
 
+void WindowObserver::OnEmbeddedAppDisconnected(Window* window) {}
+
 }  // namespace aura