Add remaining trivial targets for the Linux GN build

sync/tools/sync_tools.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.
 
 {
   'variables': {
     'chromium_code': 1,
   },
 
   'targets': [
   {
+    # GN: //sync/tools:common
     'target_name': 'sync_tools_helper',
     'type': 'static_library',
     'include_dirs': [
       '../..',
     ],
     'dependencies': [
       '../../base/base.gyp:base',
       '../../components/components.gyp:invalidation',
       '../sync.gyp:sync',
     ],
     'export_dependent_settings': [
       '../../base/base.gyp:base',
       '../sync.gyp:sync',
     ],
     'sources': [
       'invalidation_helper.cc',
       'invalidation_helper.h',
       'null_invalidation_state_tracker.cc',
       'null_invalidation_state_tracker.h',
     ],
   },
   # A tool to listen to sync notifications and print them out.
   {
+    # GN: //sync/tools:sync_listen_notifications
     'target_name': 'sync_listen_notifications',
     'type': 'executable',
     'defines': [
       'SYNC_TEST',
     ],
     'dependencies': [
       '../../base/base.gyp:base',
       '../../components/components.gyp:invalidation',
       '../../jingle/jingle.gyp:notifier',
       '../../net/net.gyp:net',
       '../../net/net.gyp:net_test_support',
       '../sync.gyp:sync',
       'sync_tools_helper',
     ],
     'sources': [
       'sync_listen_notifications.cc',
     ],
   },
 
   # A standalone command-line sync client.
   {
+    # GN: //sync/tools:sync_client
     'target_name': 'sync_client',
     'type': 'executable',
     'defines': [
       'SYNC_TEST',
     ],
     'dependencies': [
       '../../base/base.gyp:base',
       '../../components/components.gyp:invalidation',
       '../../jingle/jingle.gyp:notifier',
       '../../net/net.gyp:net',
       '../../net/net.gyp:net_test_support',
       '../sync.gyp:sync',
       '../sync.gyp:test_support_sync_core',
       'sync_tools_helper',
     ],
     'sources': [
       'sync_client.cc',
     ],
   },
   ]
 }