[Sync] Add support for proxy types

sync/syncable/model_type.cc

Index: sync/syncable/model_type.cc
diff --git a/sync/syncable/model_type.cc b/sync/syncable/model_type.cc
index 1b0bee44cc32ecc7731da35ca4b6b8444b71661a..40fc522f050a994c74a846a89a280d11ff9fab59 100644
--- a/sync/syncable/model_type.cc
+++ b/sync/syncable/model_type.cc
@@ -305,6 +305,12 @@ ModelTypeSet ControlTypes() {
   return set;
 }
 
+ModelTypeSet LocalTypes() {
+  ModelTypeSet set;
+  // TODO(zea): add a TABS type here.
+  return set;
+}
+
 bool IsControlType(ModelType model_type) {
   return ControlTypes().Has(model_type);
 }