Switched sync protobufs to use "heavy" protobufs.

chrome/browser/sync/protocol/preference_specifics.proto

 // Copyright (c) 2010 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.
 //
 // Sync protocol datatype extension for preferences.
 
 syntax = "proto2";
 
-option optimize_for = LITE_RUNTIME;
+// TODO(akalin): Re-enable this once LITE_RUNTIME supports preserving
+// unknown fields.
+
+// option optimize_for = LITE_RUNTIME;
 
 package sync_pb;
 
 import "sync.proto";
 
 // Properties of preference sync objects.
 message PreferenceSpecifics {
   optional string name = 1;
   optional string value = 2;
 }
 
 extend EntitySpecifics {
   optional PreferenceSpecifics preference = 37702;
 }