sync: Add ControlPreference protobuf and supporting code

sync/internal_api/public/base_node.h

Index: sync/internal_api/public/base_node.h
diff --git a/sync/internal_api/public/base_node.h b/sync/internal_api/public/base_node.h
index b3946fb17c6af85c781bccdf711f41a2fcea73c2..360292353a23ccf128b98712141fcaefab5c6f95 100644
--- a/sync/internal_api/public/base_node.h
+++ b/sync/internal_api/public/base_node.h
@@ -161,6 +161,11 @@ class SYNC_EXPORT BaseNode {
   // data.  Can only be called if GetModelType() == EXPERIMENTS.
   const sync_pb::ExperimentsSpecifics& GetExperimentsSpecifics() const;
 
+  // Getter specific to the PRIORITY_PREFERENCE datatype. Returns protobuf
+  // data.  Can only be called if GetModelType() == PRIORITY_PREFERENCE.
+  const sync_pb::PriorityPreferenceSpecifics&
+      GetPriorityPreferenceSpecifics() const;
+
   const sync_pb::EntitySpecifics& GetEntitySpecifics() const;
 
   // Returns the local external ID associated with the node.