aura: Reland r266681 to debug WindowObserver crashes.

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 54 matching lines @@
         'remote_window_tree_host_win.cc',       
         'remote_window_tree_host_win.h',
         'scoped_window_targeter.cc',
         'scoped_window_targeter.h',
         'window.cc',
         'window.h',
         'window_event_dispatcher.cc',
         'window_event_dispatcher.h',
         'window_delegate.h',
         'window_layer_type.h',
+        'window_observer.cc',
         'window_observer.h',
         'window_targeter.cc',
         'window_targeter.h',
         'window_tracker.cc',
         'window_tracker.h',
         'window_tree_host.cc',
         'window_tree_host.h',
         'window_tree_host_mac.mm',
         'window_tree_host_mac.h',
         'window_tree_host_observer.h',
@@ skipping 207 matching lines @@
         ['OS=="linux" and use_allocator!="none"', {
           'dependencies': [
            # See http://crbug.com/162998#c4 for why this is needed.
             '../../base/allocator/allocator.gyp:allocator',
           ],
         }],
       ],
     },
   ],
 }