Make chrome/common compile in GN

sync/sync.gyp

 # 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.
 
 {
   'variables': {
     'chromium_code': 1,
   },
 
   'includes': [
@@ skipping 86 matching lines @@
           'target_name': 'sync_notifier',
           'type': 'static_library',
           'variables': { 'enable_wexit_time_destructors': 1, },
           'includes': [
             'sync_notifier.gypi',
           ],
         },
 
         # The sync protocol buffer library.
         {
+          # GN version: //sync/protocol
           'target_name': 'sync_proto',
           'type': 'static_library',
           'variables': { 'enable_wexit_time_destructors': 1, },
           'includes': [
             'sync_proto.gypi',
           ],
         },
       ],
     },
     {  # component != static_library
       'targets': [
         # The public sync shared library target.
         {
           'target_name': 'sync',
           'type': 'shared_library',
           'variables': { 'enable_wexit_time_destructors': 1, },
           'includes': [
             'sync_api.gypi',
             'sync_core.gypi',
             'sync_internal_api.gypi',
             'sync_notifier.gypi',
             'sync_proto.gypi',
           ],
         },
       ],
     }],
   ],
 }