aura-mus: Implement stacking in DesktopWindowTreeHostMus.

services/ui/ws/access_policy_delegate.h

Index: services/ui/ws/access_policy_delegate.h
diff --git a/services/ui/ws/access_policy_delegate.h b/services/ui/ws/access_policy_delegate.h
index bd654f795681ccae83e003366cf6e6555f1c940b..34d81dec3137e2bf6aafffc43f3b2fc322ca8355 100644
--- a/services/ui/ws/access_policy_delegate.h
+++ b/services/ui/ws/access_policy_delegate.h
@@ -30,6 +30,10 @@ class AccessPolicyDelegate {
   virtual bool IsWindowRootOfAnotherTreeForAccessPolicy(
       const ServerWindow* window) const = 0;
 
+  // Returns true if |window| was created by the window manager.
+  virtual bool IsWindowCreatedByWindowManager(
+      const ServerWindow* window) const = 0;
+
  protected:
   virtual ~AccessPolicyDelegate() {}
 };