[Extensions] Pull ExtensionId into its own file

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 36 matching lines @@
       'common/error_utils.cc',
       'common/error_utils.h',
       'common/event_filter.cc',
       'common/event_filter.h',
       'common/event_filtering_info.cc',
       'common/event_filtering_info.h',
       'common/event_matcher.cc',
       'common/event_matcher.h',
       'common/extension.cc',
       'common/extension.h',
+      'common/extension_id.h',
       'common/extension_api.cc',
       'common/extension_api.h',
       'common/extension_icon_set.cc',
       'common/extension_icon_set.h',
       'common/extension_l10n_util.cc',
       'common/extension_l10n_util.h',
       'common/extension_message_generator.cc',
       'common/extension_message_generator.h',
       'common/extension_messages.cc',
       'common/extension_messages.h',
@@ skipping 1068 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',
     ],
   },
 }