[Sync] Add support for proxy types

sync/syncable/directory_backing_store.cc

Index: sync/syncable/directory_backing_store.cc
diff --git a/sync/syncable/directory_backing_store.cc b/sync/syncable/directory_backing_store.cc
index 442c9e7d5518bae1cc787590089ff35e67ab9f4f..e3352d89814a4abb1028c85fc783f244e1eb8ee1 100644
--- a/sync/syncable/directory_backing_store.cc
+++ b/sync/syncable/directory_backing_store.cc
@@ -266,8 +266,11 @@ bool DirectoryBackingStore::SaveChanges(
             "VALUES (?, ?, ?)"));
 
     for (int i = FIRST_REAL_MODEL_TYPE; i < MODEL_TYPE_COUNT; ++i) {
+      ModelType type = ModelTypeFromInt(i);
+      if (VirtualTypes().Has(type))
+        continue;
       // We persist not ModelType but rather a protobuf-derived ID.
-      string model_id = ModelTypeEnumToModelId(ModelTypeFromInt(i));
+      string model_id = ModelTypeEnumToModelId(type);
       string progress_marker;
       info.download_progress[i].SerializeToString(&progress_marker);
       s2.BindBlob(0, model_id.data(), model_id.length());