Inline extensions .gypi files into GN build.

extensions/shell/browser/api/api_registration.gyp

-# Copyright 2014 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: //extensions/shell/common/api:api_registration
-      'target_name': 'shell_api_registration',
-      'type': 'static_library',
-      # TODO(jschuh): http://crbug.com/167187 size_t -> int
-      'msvs_disabled_warnings': [ 4267 ],
-      'includes': [
-        '../../../../build/json_schema_bundle_registration_compile.gypi',
-        '../../common/api/schemas.gypi',
-      ],
-      'dependencies': [
-        '<(DEPTH)/extensions/shell/common/api/api.gyp:shell_api',
-      ],
-    },
-  ],
-}