views: Extract Widget observer into its own header file.

ui/views/views.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,
   },
 
   'target_defaults': {
     'conditions': [
@@ skipping 332 matching lines @@
         'widget/native_widget_win.cc',
         'widget/native_widget_win.h',
         'widget/widget.cc',
         'widget/widget.h',
         'widget/widget_delegate.cc',
         'widget/widget_delegate.h',
         'widget/widget_hwnd_utils.cc',
         'widget/widget_hwnd_utils.h',
         'widget/widget_message_filter.cc',
         'widget/widget_message_filter.h',
+        'widget/widget_observer.h',
         'widget/x11_desktop_handler.cc',
         'widget/x11_desktop_handler.h',
         'widget/x11_window_event_filter.cc',
         'widget/x11_window_event_filter.h',
         'window/client_view.cc',
         'window/client_view.h',
         'window/custom_frame_view.cc',
         'window/custom_frame_view.h',
         'window/dialog_client_view.cc',
         'window/dialog_client_view.h',
@@ skipping 366 matching lines @@
           'sources/': [
             # This is needed because the aura rule strips it from the default
             # sources list.
             ['include', '^../../content/app/startup_helper_win.cc'],
           ],
         }],
       ],
     },  # target_name: views_examples_lib
   ],
 }