[Sync] Replace uses of ObserverListThreadSafe with WeakHandles

chrome/chrome_tests.gypi

 # Copyright (c) 2011 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' : {
     # Variables for js2gtest rules
     'gypv8sh': '../tools/gypv8sh.py',
     'js2gtest': 'test/base/js2gtest.js',
     'mock_js': 'third_party/mock4js/mock4js.js',
     'test_api_js': 'test/data/webui/test_api.js',
@@ skipping 336 matching lines @@
         'sync',
       ],
       'include_dirs': [
         '..',
       ],
       'sources': [
         'browser/sync/js/js_test_util.cc',
         'browser/sync/js/js_test_util.h',
         'browser/sync/test/null_directory_change_delegate.cc',
         'browser/sync/test/null_directory_change_delegate.h',
+        'browser/sync/test/null_transaction_observer.cc',
+        'browser/sync/test/null_transaction_observer.h',
         'browser/sync/test/engine/test_directory_setter_upper.cc',
         'browser/sync/test/engine/test_directory_setter_upper.h',
       ],
     },
     {
       'target_name': 'test_support_syncapi',
       'type': 'static_library',
       'dependencies': [
         '../base/base.gyp:base',
         'syncapi_core',
@@ skipping 3906 matching lines @@
               # Use outputs of this action as inputs for the main target build.
               # Seems as a misnomer but makes this happy on Linux (scons).
               'process_outputs_as_sources': 1,
             },
           ],  # 'actions'
         },
       ]
     }],  # 'coverage!=0'
   ],  # 'conditions'
 }