Add SessionStateObserver with ActiveUserChanged()

ash/ash.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,
     'grit_out_dir': '<(SHARED_INTERMEDIATE_DIR)/chrome',
   },
   'includes': [
@@ skipping 155 matching lines @@
         'root_window_controller.cc',
         'root_window_controller.h',
         'rotator/screen_rotation.cc',
         'rotator/screen_rotation.h',
         'screen_ash.cc',
         'screen_ash.h',
         'screensaver/screensaver_view.cc',
         'screensaver/screensaver_view.h',
         'screenshot_delegate.h',
         'session_state_delegate.h',
+        'session_state_observer.h',
         'shelf/background_animator.cc',
         'shelf/background_animator.h',
         'shelf/shelf_layout_manager.cc',
         'shelf/shelf_layout_manager.h',
         'shelf/shelf_layout_manager_observer.h',
         'shelf/shelf_types.h',
         'shelf/shelf_widget.cc',
         'shelf/shelf_widget.h',
         'shell.cc',
         'shell.h',
@@ skipping 668 matching lines @@
             'shell/cocoa/app-Info.plist',
           ],
           'xcode_settings': {
             'INFOPLIST_FILE': 'shell/cocoa/app-Info.plist',
           },
         }],
       ],
     },
   ],
 }