aura-mus: Implement stacking in DesktopWindowTreeHostMus.

services/ui/ws/window_manager_access_policy.cc

Index: services/ui/ws/window_manager_access_policy.cc
diff --git a/services/ui/ws/window_manager_access_policy.cc b/services/ui/ws/window_manager_access_policy.cc
index 862402749d174b489d9a7377b075af4ade2ca352..e38c7ba892d1530ce45479950395d707e3b46959 100644
--- a/services/ui/ws/window_manager_access_policy.cc
+++ b/services/ui/ws/window_manager_access_policy.cc
@@ -142,6 +142,14 @@ bool WindowManagerAccessPolicy::CanSetAcceptEvents(
          delegate_->HasRootForAccessPolicy(window);
 }
 
+bool WindowManagerAccessPolicy::CanStackAtTop(
+    const ServerWindow* window) const {
+  // This API is for clients. Window managers can perform any arbitrary
+  // reordering of the windows and don't need to go through this constrained
+  // API.
+  return false;
+}
+
 bool WindowManagerAccessPolicy::CanSetCursorProperties(
     const ServerWindow* window) const {
   return WasCreatedByThisClient(window) ||