sync: Add ControlPreference protobuf and supporting code

sync/protocol/priority_preference_specifics.proto

Index: sync/protocol/priority_preference_specifics.proto
diff --git a/sync/protocol/preference_specifics.proto b/sync/protocol/priority_preference_specifics.proto
similarity index 75%
copy from sync/protocol/preference_specifics.proto
copy to sync/protocol/priority_preference_specifics.proto
index 4827c5dc20ceba8b127ce45bb52d0d7ee6f9c0d5..e52e0befa03b9f47b34b1c80b841327b506c5c4d 100644
--- a/sync/protocol/preference_specifics.proto
+++ b/sync/protocol/priority_preference_specifics.proto
@@ -2,7 +2,7 @@
 // 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.
@@ -14,9 +14,8 @@ 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;
 }
-