aura-mus: Implement stacking in DesktopWindowTreeHostMus.

services/ui/ws/default_access_policy.cc

Index: services/ui/ws/default_access_policy.cc
diff --git a/services/ui/ws/default_access_policy.cc b/services/ui/ws/default_access_policy.cc
index 0acee434e1f5f47e05f9f8e971637d7386a2bbcd..5b65ab8ac3c52f24f354106bfbf40be8d6f54b04 100644
--- a/services/ui/ws/default_access_policy.cc
+++ b/services/ui/ws/default_access_policy.cc
@@ -156,6 +156,11 @@ bool DefaultAccessPolicy::CanSetAcceptEvents(const ServerWindow* window) const {
          delegate_->HasRootForAccessPolicy(window);
 }
 
+bool DefaultAccessPolicy::CanStackAtTop(const ServerWindow* window) const {
+  return delegate_->HasRootForAccessPolicy(window) &&
+         delegate_->IsWindowCreatedByWindowManager(window);
+}
+
 bool DefaultAccessPolicy::CanSetCursorProperties(
     const ServerWindow* window) const {
   return WasCreatedByThisClient(window) ||