Enable Null Syncable ID which is different than Root ID.

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 6d08b75c1bebe271c0da86ee2c5a1b7b0e490246..536af353de60deb50cc62524f3d01dc42ec7a2ad 100644
--- a/sync/syncable/directory_backing_store.cc
+++ b/sync/syncable/directory_backing_store.cc
@@ -1548,9 +1548,11 @@ bool DirectoryBackingStore::VerifyReferenceIntegrity(
   for (Directory::MetahandlesMap::const_iterator it = handles_map->begin();
        it != handles_map->end(); ++it) {
     EntryKernel* entry = it->second;
-    bool parent_exists = (ids_set.find(entry->ref(PARENT_ID).value()) != end);
-    if (!parent_exists) {
-      return false;
+    if (!entry->ref(PARENT_ID).IsNull()) {
+      bool parent_exists = (ids_set.find(entry->ref(PARENT_ID).value()) != end);
+      if (!parent_exists) {
+        return false;
+      }
     }
   }
   return is_ok;