sync: Add ControlPreference protobuf and supporting code

sync/internal_api/base_node.cc

Index: sync/internal_api/base_node.cc
diff --git a/sync/internal_api/base_node.cc b/sync/internal_api/base_node.cc
index f30c36e2f244bcdfd035ddc4f868f5cf509915cf..4ae318ba9f9078b14c33308ae33b4d31a94aa4f5 100644
--- a/sync/internal_api/base_node.cc
+++ b/sync/internal_api/base_node.cc
@@ -343,6 +343,12 @@ const sync_pb::ExperimentsSpecifics& BaseNode::GetExperimentsSpecifics() const {
   return GetEntitySpecifics().experiments();
 }
 
+const sync_pb::PriorityPreferenceSpecifics&
+    BaseNode::GetPriorityPreferenceSpecifics() const {
+  DCHECK_EQ(GetModelType(), PRIORITY_PREFERENCES);
+  return GetEntitySpecifics().priority_preference();
+}
+
 const sync_pb::EntitySpecifics& BaseNode::GetEntitySpecifics() const {
   return GetUnencryptedSpecifics(GetEntry());
 }