Add browser_extensions to GN build.

extensions/common/api/api.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/common/api
       'target_name': 'extensions_api',
       'type': 'static_library',
       'sources': [
         '<@(schema_files)',
       ],
       # TODO(jschuh): http://crbug.com/167187 size_t -> int
       'msvs_disabled_warnings': [ 4267 ],
       'includes': [
         '../../../build/json_schema_bundle_compile.gypi',
         '../../../build/json_schema_compile.gypi',
       ],
       'variables': {
         'chromium_code': 1,
         'non_compiled_schema_files': [
         ],
         'conditions': [
           ['enable_extensions==1', {
+            # Note: file list duplicated in GN build.
             'schema_files': [
               'app_runtime.idl',
               'app_view_internal.json',
               'dns.idl',
               'extensions_manifest_types.json',
               'power.idl',
               'runtime.json',
               'serial.idl',
               'socket.idl',
               'sockets_tcp.idl',
@@ skipping 15 matching lines @@
         'root_namespace': 'extensions::core_api',
         'impl_dir': 'extensions/browser/api',
       },
       'dependencies': [
         '<(DEPTH)/device/serial/serial.gyp:device_serial',
         '<(DEPTH)/skia/skia.gyp:skia',
       ],
     },
   ],
 }