exo: Implement notification surface support

components/exo.gypi

 # Copyright 2015 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.
 
 {
   'targets': [
     {
       # GN version: //components/exo
       'target_name': 'exo',
       'type': 'static_library',
@@ skipping 20 matching lines @@
       ],
       'sources': [
         # Note: sources list duplicated in GN build.
         'exo/buffer.cc',
         'exo/buffer.h',
         'exo/display.cc',
         'exo/display.h',
         'exo/keyboard.cc',
         'exo/keyboard.h',
         'exo/keyboard_delegate.h',
+        'exo/notification_surface.cc',
+        'exo/notification_surface.h',
+        'exo/notification_surface_registry.cc',
+        'exo/notification_surface_registry.h',
         'exo/pointer.cc',
         'exo/pointer.h',
         'exo/pointer_delegate.h',
         'exo/shared_memory.cc',
         'exo/shared_memory.h',
         'exo/shell_surface.cc',
         'exo/shell_surface.h',
         'exo/sub_surface.cc',
         'exo/sub_surface.h',
         'exo/surface.cc',
@@ skipping 55 matching lines @@
               'defines': [
                 'USE_XKBCOMMON',
               ],
             }],
           ],
         },
       ],
     }],
   ],
 }