[ash-md] Prevents auto-positioning after cancelling overview

ash/common/wm/overview/window_selector.h

 // Copyright 2013 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.
 
 #ifndef ASH_COMMON_WM_OVERVIEW_WINDOW_SELECTOR_H_
 #define ASH_COMMON_WM_OVERVIEW_WINDOW_SELECTOR_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 163 matching lines @@
   size_t num_times_textfield_cleared_;
 
   // Tracks whether minimized windows are currently being restored for overview
   // mode.
   bool restoring_minimized_windows_;
 
   // The distance between the top edge of the screen and the bottom edge of
   // the text filtering textfield.
   int text_filter_bottom_;
 
+  // True once Shutdown() has been called.
+  bool shutting_down_;
+
   DISALLOW_COPY_AND_ASSIGN(WindowSelector);
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_WM_OVERVIEW_WINDOW_SELECTOR_H_