Relanding the EventTarget fixes for Aura (Ash) as the previous attempt was reverted due to interact…

ash/wm/event_client_impl.cc

 // Copyright (c) 2012 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 "ash/wm/event_client_impl.h"
 
 #include "ash/shell.h"
 #include "ash/shell_window_ids.h"
 #include "ui/aura/window.h"
 
@@ skipping 20 matching lines @@
     const aura::Window* lock_screen_related_containers = Shell::GetContainer(
         root_window,
         kShellWindowId_LockScreenRelatedContainersContainer);
     return lock_screen_containers->Contains(window) ||
         lock_background_containers->Contains(window) ||
         lock_screen_related_containers->Contains(window);
   }
   return true;
 }
 
+ui::EventTarget* EventClientImpl::GetToplevelEventTarget() {
+  return Shell::GetInstance();
+}
+
 }  // namespace internal
 }  // namespace ash