sync: Add ControlPreference protobuf and supporting code

sync/protocol/proto_value_conversions.h

Index: sync/protocol/proto_value_conversions.h
diff --git a/sync/protocol/proto_value_conversions.h b/sync/protocol/proto_value_conversions.h
index a553e56b28aa4383775b315c159520d2a5672174..9003194e9fbb4c4341b1035a9bfe6145642f20e7 100644
--- a/sync/protocol/proto_value_conversions.h
+++ b/sync/protocol/proto_value_conversions.h
@@ -43,6 +43,7 @@ class NigoriSpecifics;
 class PasswordSpecifics;
 class PasswordSpecificsData;
 class PreferenceSpecifics;
+class PriorityPreferenceSpecifics;
 class SearchEngineSpecifics;
 class SessionHeader;
 class SessionSpecifics;
@@ -141,6 +142,9 @@ SYNC_EXPORT_PRIVATE base::DictionaryValue* DeviceInfoSpecificsToValue(
 SYNC_EXPORT_PRIVATE base::DictionaryValue* ExperimentsSpecificsToValue(
     const sync_pb::ExperimentsSpecifics& proto);
 
+SYNC_EXPORT_PRIVATE base::DictionaryValue* PriorityPreferenceSpecificsToValue(
+    const sync_pb::PriorityPreferenceSpecifics& proto);
+
 SYNC_EXPORT_PRIVATE base::DictionaryValue* ExtensionSettingSpecificsToValue(
     const sync_pb::ExtensionSettingSpecifics& extension_setting_specifics);