Add WorkspaceObserver to observe changes in workspace state

ui/aura_shell/aura_shell.gyp

 # Copyright (c) 2011 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 58 matching lines @@
         'status_area_view.cc',
         'status_area_view.h',
         'toplevel_frame_view.cc',
         'toplevel_frame_view.h',
         'workspace/workspace.cc',
         'workspace/workspace.h',
         'workspace/workspace_controller.cc',
         'workspace/workspace_controller.h',
         'workspace/workspace_manager.cc',
         'workspace/workspace_manager.h',
+        'workspace/workspace_observer.h',
       ],
     },
     {
       'target_name': 'aura_shell_unittests',
       'type': 'executable',
       'dependencies': [
         '../../base/base.gyp:base',
         '../../base/base.gyp:test_support_base',
         '../../build/temp_gyp/googleurl.gyp:googleurl',
         '../../skia/skia.gyp:skia',
@@ skipping 85 matching lines @@
         '../../views/views.gyp:views_desktop',
         '../../views/views.gyp:views_desktop_lib',
         '../../views/views.gyp:views_unittests',
         '../aura/aura.gyp:*',
         '../gfx/compositor/compositor.gyp:*',
         'aura_shell_exe',
       ],
     },
   ],
 }