mus: Do not notify clients when the roots are reordered.

components/mus/ws/window_tree_impl.cc

Index: components/mus/ws/window_tree_impl.cc
diff --git a/components/mus/ws/window_tree_impl.cc b/components/mus/ws/window_tree_impl.cc
index 80690ca809f36efd813414863b43e11d8419696e..f2b01c26eeb0161f4dc929e2ce6c926ebf896b47 100644
--- a/components/mus/ws/window_tree_impl.cc
+++ b/components/mus/ws/window_tree_impl.cc
@@ -409,11 +409,16 @@ void WindowTreeImpl::ProcessWindowReorder(const ServerWindow* window,
                                           const ServerWindow* relative_window,
                                           mojom::OrderDirection direction,
                                           bool originated_change) {
+  DCHECK_EQ(window->parent(), relative_window->parent());
   if (originated_change || !IsWindowKnown(window) ||
       !IsWindowKnown(relative_window) ||
       connection_manager_->DidConnectionMessageClient(id_))
     return;
 
+  // Do not notify ordering changes of the root windows.

[sky, 2016/01/12 22:36:08]

Document why. If window or relative_window is a root then the client can't see
above the window and so doesn't need to know about the change.

[sadrul, 2016/01/13 02:26:05]

Done.

+  if (HasRoot(window) || HasRoot(relative_window))
+    return;
+
   client_->OnWindowReordered(MapWindowIdToClient(window),
                              MapWindowIdToClient(relative_window), direction);
   connection_manager_->OnConnectionMessagedClient(id_);