Converts WindowTreeClientTest to be in terms of aura

ui/views/mus/mus_client.cc

 // Copyright 2016 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/views/mus/mus_client.h"
 
 #include "base/bind.h"
 #include "base/memory/ptr_util.h"
 #include "services/service_manager/public/cpp/connection.h"
 #include "services/service_manager/public/cpp/connector.h"
@@ skipping 194 matching lines @@
       base::Bind(&MusClient::CreateNativeWidget, base::Unretained(this)));
 }
 
 void MusClient::OnEmbed(
     std::unique_ptr<aura::WindowTreeHostMus> window_tree_host) {
   NOTREACHED();
 }
 
 void MusClient::OnLostConnection(aura::WindowTreeClient* client) {}
 
-void MusClient::OnEmbedRootDestroyed(aura::Window* root) {
-  // Not called for MusClient as WindowTreeClient isn't created by
-  // way of an Embed().
-  NOTREACHED();
+void MusClient::OnEmbedRootDestroyed(
+    aura::WindowTreeHostMus* window_tree_host) {
+  static_cast<DesktopWindowTreeHostMus*>(window_tree_host)
+      ->ServerDestroyedWindow();
 }
 
 void MusClient::OnPointerEventObserved(const ui::PointerEvent& event,
                                        aura::Window* target) {
   pointer_watcher_event_router_->OnPointerEventObserved(event, target);
 }
 
 void MusClient::OnWindowManagerFrameValuesChanged() {
   for (auto& observer : observer_list_)
     observer.OnWindowManagerFrameValuesChanged();
@@ skipping 23 matching lines @@
       return root->GetTopWindowContainingPoint(relative_point);
   }
   return nullptr;
 }
 
 std::unique_ptr<OSExchangeData::Provider> MusClient::BuildProvider() {
   return base::MakeUnique<aura::OSExchangeDataProviderMus>();
 }
 
 }  // namespace views