Converts ash/wm/mru_window_tracker_unittest.cc to use AshTest

ash/mus/bridge/wm_shell_mus.cc

 // Copyright 2016 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/mus/bridge/wm_shell_mus.h"
 
 #include <utility>
 
 #include "ash/common/accelerators/accelerator_controller.h"
 #include "ash/common/key_event_watcher.h"
@@ skipping 435 matching lines @@
                        WmWindowMus::Get(window)->GetShellWindowId());
 }
 
 // TODO: support OnAttemptToReactivateWindow, http://crbug.com/615114.
 void WmShellMus::OnWindowTreeFocusChanged(ui::Window* gained_focus,
                                           ui::Window* lost_focus) {
   WmWindow* gained_active = GetToplevelAncestor(gained_focus);
   if (gained_active)
     set_root_window_for_new_windows(gained_active->GetRootWindow());
 
-  WmWindow* lost_active = GetToplevelAncestor(gained_focus);
+  WmWindow* lost_active = GetToplevelAncestor(lost_focus);
   if (gained_active == lost_active)
     return;
 
   FOR_EACH_OBSERVER(WmActivationObserver, activation_observers_,
                     OnWindowActivated(gained_active, lost_active));
 }
 
 void WmShellMus::OnDidDestroyClient(ui::WindowTreeClient* client) {
   DCHECK_EQ(window_tree_client(), client);
   client->RemoveObserver(this);
 }
 
 }  // namespace mus
 }  // namespace ash