Bug 1184400, part 3 - Use HasExternalReference in XPCWrappedNative::Traverse. r=bholley

This is just for consistency.

js/xpconnect/src/XPCWrappedNative.cpp

@@ -68,7 +68,7 @@ NS_CYCLE_COLLECTION_CLASSNAME(XPCWrappedNative)::Traverse
         NS_IMPL_CYCLE_COLLECTION_DESCRIBE(XPCWrappedNative, tmp->mRefCnt.get())
     }
 
-    if (tmp->mRefCnt.get() > 1) {
+    if (tmp->HasExternalReference()) {
 
         // If our refcount is > 1, our reference to the flat JS object is
         // considered "strong", and we're going to traverse it.