[Extensions UI] Add a placeholder icon for extensions in the toolbar

extensions/extensions.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.
 
 {
   'variables': {
     'extensions_common_constants_sources': [
       'common/constants.cc',
       'common/constants.h',
     ],
@@ skipping 560 matching lines @@
       'browser/extension_function_registry.h',
       'browser/extension_function_util.cc',
       'browser/extension_function_util.h',
       'browser/extension_host.cc',
       'browser/extension_host.h',
       'browser/extension_host_delegate.h',
       'browser/extension_host_observer.h',
       'browser/extension_host_queue.h',
       'browser/extension_icon_image.cc',
       'browser/extension_icon_image.h',
+      'browser/extension_icon_placeholder.cc',
+      'browser/extension_icon_placeholder.h',
       'browser/extension_message_filter.cc',
       'browser/extension_message_filter.h',
       'browser/extension_pref_store.cc',
       'browser/extension_pref_store.h',
       'browser/extension_pref_value_map.cc',
       'browser/extension_pref_value_map.h',
       'browser/extension_pref_value_map_factory.cc',
       'browser/extension_pref_value_map_factory.h',
       'browser/extension_prefs.cc',
       'browser/extension_prefs.h',
@@ skipping 443 matching lines @@
       'test/result_catcher.h',
       'test/test_content_utility_client.cc',
       'test/test_content_utility_client.h',
       'test/test_extensions_client.cc',
       'test/test_extensions_client.h',
       'test/test_permission_message_provider.cc',
       'test/test_permission_message_provider.h',
     ],
   },
 }