Changes how window manager obtains WindowTree from mus

ash/mus/bridge/wm_window_mus.h

 // 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.
 
 #ifndef ASH_MUS_BRIDGE_WM_WINDOW_MUS_H_
 #define ASH_MUS_BRIDGE_WM_WINDOW_MUS_H_
 
 #include <memory>
 
 #include "ash/common/shell_window_ids.h"
@@ skipping 79 matching lines @@
   }
   static const WmWindowMus* AsWmWindowMus(const WmWindow* window) {
     return static_cast<const WmWindowMus*>(window);
   }
 
   wm::WindowState* GetWindowState() { return WmWindow::GetWindowState(); }
 
   // See description of |children_use_extended_hit_region_|.
   bool ShouldUseExtendedHitRegion() const;
 
+  // Returns true if this window is considered a shell window container.
+  bool IsContainer() const;
+
   // WmWindow:
   const WmWindow* GetRootWindow() const override;
   WmRootWindowController* GetRootWindowController() override;
   WmShell* GetShell() const override;
   void SetName(const char* name) override;
   base::string16 GetTitle() const override;
   void SetShellWindowId(int id) override;
   int GetShellWindowId() const override;
   WmWindow* GetChildByShellWindowId(int id) override;
   ui::wm::WindowType GetType() const override;
@@ skipping 140 matching lines @@
   // resizing easier.
   bool children_use_extended_hit_region_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(WmWindowMus);
 };
 
 }  // namespace mus
 }  // namespace ash
 
 #endif  // ASH_MUS_BRIDGE_WM_WINDOW_MUS_H_