Add AttachmentDownloader interface, change signature of AttachmentStore::Read

sync/sync_api.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',
   ],
   'dependencies': [
     '../base/base.gyp:base',
   ],
   'sources': [
     'api/attachments/attachment.cc',
     'api/attachments/attachment.h',
+    'api/attachments/attachment_downloader.cc',
+    'api/attachments/attachment_downloader.h',
     'api/attachments/attachment_id.cc',
     'api/attachments/attachment_id.h',
     'api/attachments/attachment_service.cc',
     'api/attachments/attachment_service.h',
     'api/attachments/attachment_service_impl.cc',
     'api/attachments/attachment_service_impl.h',
     'api/attachments/attachment_service_proxy.cc',
     'api/attachments/attachment_service_proxy.h',
     'api/attachments/attachment_service_proxy_for_test.cc',
     'api/attachments/attachment_service_proxy_for_test.h',
@@ skipping 12 matching lines @@
     'api/sync_change_processor.h',
     'api/sync_error.cc',
     'api/sync_error.h',
     'api/sync_error_factory.cc',
     'api/sync_error_factory.h',
     'api/sync_merge_result.cc',
     'api/sync_merge_result.h',
     'api/time.h',
   ],
 }