sync: Add ControlPreference protobuf and supporting code

sync/protocol/priority_preference_specifics.proto

 // 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.
 //
-// Sync protocol datatype extension for preferences.
+// Sync protocol datatype extension for priority preferences.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 option retain_unknown_fields = true;
 
 package sync_pb;
 
-// Properties of preference sync objects.
-message PreferenceSpecifics {
+// Properties of a synced priority preference.
+message PriorityPreferenceSpecifics {
   optional string name = 1;
   optional string value = 2;
 }
-