Changes how window manager obtains WindowTree from mus

ash/mus/bridge/wm_window_mus.h

Index: ash/mus/bridge/wm_window_mus.h
diff --git a/ash/mus/bridge/wm_window_mus.h b/ash/mus/bridge/wm_window_mus.h
index c7a907e58205b326a8e03cf34c0fa925247c039d..2f17ee2a24041d9ec66f94c4060082241fd80f9f 100644
--- a/ash/mus/bridge/wm_window_mus.h
+++ b/ash/mus/bridge/wm_window_mus.h
@@ -97,6 +97,9 @@ class WmWindowMus : public WmWindow, public ::mus::WindowObserver {
   // 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;