Make chrome/common compile in GN

sync/sync_proto.gypi

 # Copyright 2013 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.
 
 {
   'include_dirs': [
     '..',
   ],
   'defines': [
     'SYNC_IMPLEMENTATION',
   ],
   'sources': [
+    # NOTE: If you add a file to this list, also add it to
+    # sync/protocol/BUILD.gn
     'protocol/app_notification_specifics.proto',
     'protocol/app_setting_specifics.proto',
     'protocol/app_specifics.proto',
     'protocol/app_list_specifics.proto',
     'protocol/article_specifics.proto',
     'protocol/attachments.proto',
     'protocol/autofill_specifics.proto',
     'protocol/bookmark_specifics.proto',
     'protocol/client_commands.proto',
     'protocol/client_debug_info.proto',
@@ skipping 30 matching lines @@
   'variables': {
     'proto_in_dir': './protocol',
     'proto_out_dir': 'sync/protocol',
     'cc_generator_options': 'dllexport_decl=SYNC_EXPORT:',
     'cc_include': 'sync/base/sync_export.h',
   },
   'includes': [
     '../build/protoc.gypi'
   ],
 }