Changes how window manager obtains WindowTree from mus

ash/mus/bridge/wm_window_mus.cc

Index: ash/mus/bridge/wm_window_mus.cc
diff --git a/ash/mus/bridge/wm_window_mus.cc b/ash/mus/bridge/wm_window_mus.cc
index 09b0c06e3218993abc411a863ece7f91f161e699..c822398b79090c3c28d5d8a49f0d539e9a9decdf 100644
--- a/ash/mus/bridge/wm_window_mus.cc
+++ b/ash/mus/bridge/wm_window_mus.cc
@@ -155,6 +155,10 @@ bool WmWindowMus::ShouldUseExtendedHitRegion() const {
   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());
 }