Changes how window manager obtains WindowTree from mus

ash/mus/bridge/wm_window_mus.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 "ash/mus/bridge/wm_window_mus.h"
 
 #include "ash/common/wm/container_finder.h"
 #include "ash/common/wm/window_state.h"
 #include "ash/common/wm_layout_manager.h"
 #include "ash/common/wm_window_observer.h"
@@ skipping 137 matching lines @@
 const WmRootWindowControllerMus* WmWindowMus::GetRootWindowControllerMus()
     const {
   return WmRootWindowControllerMus::Get(window_->GetRoot());
 }
 
 bool WmWindowMus::ShouldUseExtendedHitRegion() const {
   const WmWindowMus* parent = Get(window_->parent());
   return parent && parent->children_use_extended_hit_region_;
 }
 
+bool WmWindowMus::IsContainer() const {
+  return GetShellWindowId() != kShellWindowId_Invalid;
+}
+
 const WmWindow* WmWindowMus::GetRootWindow() const {
   return Get(window_->GetRoot());
 }
 
 WmRootWindowController* WmWindowMus::GetRootWindowController() {
   return GetRootWindowControllerMus();
 }
 
 WmShell* WmWindowMus::GetShell() const {
   return WmShellMus::Get();
@@ skipping 591 matching lines @@
 void WmWindowMus::OnWindowDestroying(::mus::Window* window) {
   FOR_EACH_OBSERVER(WmWindowObserver, observers_, OnWindowDestroying(this));
 }
 
 void WmWindowMus::OnWindowDestroyed(::mus::Window* window) {
   FOR_EACH_OBSERVER(WmWindowObserver, observers_, OnWindowDestroyed(this));
 }
 
 }  // namespace mus
 }  // namespace ash