sync: remove use of protobuf extensions in protocol to reduce static init overhead.

chrome/browser/sync/syncable/syncable.cc

 // Copyright (c) 2012 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 "chrome/browser/sync/syncable/syncable.h"
 
 #include <algorithm>
 #include <cstring>
 #include <functional>
 #include <iomanip>
@@ skipping 438 matching lines @@
   for (int i = FIRST_REAL_MODEL_TYPE; i < MODEL_TYPE_COUNT; ++i) {
     reset_download_progress(ModelTypeFromInt(i));
   }
 }
 
 Directory::PersistedKernelInfo::~PersistedKernelInfo() {}
 
 void Directory::PersistedKernelInfo::reset_download_progress(
     ModelType model_type) {
   download_progress[model_type].set_data_type_id(
-      GetExtensionFieldNumberFromModelType(model_type));
+      GetSpecificsFieldNumberFromModelType(model_type));
   // An empty-string token indicates no prior knowledge.
   download_progress[model_type].set_token(std::string());
 }
 
 Directory::SaveChangesSnapshot::SaveChangesSnapshot()
     : kernel_info_status(KERNEL_SHARE_INFO_INVALID) {
 }
 
 Directory::SaveChangesSnapshot::~SaveChangesSnapshot() {}
 
@@ skipping 1900 matching lines @@
     if (entry->ref(NEXT_ID).IsRoot() ||
         entry->ref(NEXT_ID) != entry->ref(PREV_ID)) {
       return entry;
     }
   }
   // There were no children in the linked list.
   return NULL;
 }
 
 }  // namespace syncable