[Sync] Make ModelSafeWorker a true interface

chrome/chrome.gyp

 # 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': {
     'chromium_code': 1,
 
     'variables': {
       'version_py_path': 'tools/build/version.py',
       'version_path': 'VERSION',
@@ skipping 367 matching lines @@
         'browser/sync/engine/conflict_resolver.cc',
         'browser/sync/engine/conflict_resolver.h',
         'browser/sync/engine/download_updates_command.cc',
         'browser/sync/engine/download_updates_command.h',
         'browser/sync/engine/get_commit_ids_command.cc',
         'browser/sync/engine/get_commit_ids_command.h',
         'browser/sync/engine/model_changing_syncer_command.cc',
         'browser/sync/engine/model_changing_syncer_command.h',
         'browser/sync/engine/model_safe_worker.cc',
         'browser/sync/engine/model_safe_worker.h',
+        'browser/sync/engine/passive_model_worker.cc',
+        'browser/sync/engine/passive_model_worker.h',
         'browser/sync/engine/net/server_connection_manager.cc',
         'browser/sync/engine/net/server_connection_manager.h',
         'browser/sync/engine/net/url_translator.cc',
         'browser/sync/engine/net/url_translator.h',
         'browser/sync/engine/nigori_util.cc',
         'browser/sync/engine/nigori_util.h',
         'browser/sync/engine/nudge_source.cc',
         'browser/sync/engine/nudge_source.h',
         'browser/sync/engine/polling_constants.cc',
         'browser/sync/engine/polling_constants.h',
@@ skipping 920 matching lines @@
           'configurations': {
             'Common_Base': {
               'msvs_target_platform': 'x64',
             },
           },
         },
       ]},  # 'targets'
     ],  # OS=="win"
   ],  # 'conditions'
 }