Makes mus not bring window to front on activation changes

services/ui/ws/display.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "services/ui/ws/display.h"
 
 #include <set>
 #include <utility>
 #include <vector>
 
@@ skipping 333 matching lines @@
     window->DestroySurfacesScheduledForDestruction();
   }
 }
 
 bool Display::CanHaveActiveChildren(ServerWindow* window) const {
   return window && activation_parents_.Contains(window);
 }
 
 void Display::OnActivationChanged(ServerWindow* old_active_window,
                                   ServerWindow* new_active_window) {
-  DCHECK_NE(new_active_window, old_active_window);
-  if (new_active_window && new_active_window->parent()) {
-    // Raise the new active window.
-    // TODO(sad): Let the WM dictate whether to raise the window or not?
-    new_active_window->parent()->StackChildAtTop(new_active_window);
-  }
+  // Don't do anything here. We assume the window manager handles restacking. If
+  // we did attempt to restack than we would have to ensure clients see the
+  // restack.
 }
 
 void Display::OnFocusChanged(FocusControllerChangeSource change_source,
                              ServerWindow* old_focused_window,
                              ServerWindow* new_focused_window) {
   // TODO(sky): focus is global, not per windowtreehost. Move.
 
   // There are up to four clients that need to be notified:
   // . the client containing |old_focused_window|.
   // . the client with |old_focused_window| as its root.
@@ skipping 61 matching lines @@
 }
 
 void Display::OnWindowManagerWindowTreeFactoryReady(
     WindowManagerWindowTreeFactory* factory) {
   if (!binding_)
     CreateWindowManagerDisplayRootFromFactory(factory);
 }
 
 }  // namespace ws
 }  // namespace ui