Nukes aura::WindowDestructionObserver. It's effectively the same as

ui/aura/aura.gyp

 # 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.
 
 {
   'variables': {
     'chromium_code': 1,
   },
   'targets': [
     {
@@ skipping 83 matching lines @@
         'root_window_mac.h',
         'root_window_mac.mm',
         'root_window_transformer.h',
         'root_window_view_mac.h',
         'root_window_view_mac.mm',
         'root_window.cc',
         'root_window.h',
         'window.cc',
         'window.h',
         'window_delegate.h',
-        'window_destruction_observer.cc',
-        'window_destruction_observer.h',
         'window_observer.h',
         'window_tracker.cc',
         'window_tracker.h',
       ],
       'conditions': [
         ['OS=="mac"', {
           'sources/': [
             ['exclude', 'client/dispatcher_client.cc'],
             ['exclude', 'client/dispatcher_client.h'],
           ],
@@ skipping 177 matching lines @@
         ['OS=="linux" and linux_use_tcmalloc==1', {
           'dependencies': [
            # See http://crbug.com/162998#c4 for why this is needed.
             '../../base/allocator/allocator.gyp:allocator',
           ],
         }],
       ],
     },
   ],
 }