Bug 1298454 - Use element.makeWebElement instead of manually creating web element; r=automatedtester

MozReview-Commit-ID: 83W3e6mWqYJ

--HG--
extra : rebase_source : ce3ca019ef62573827a790145fe8bb4660b14bc8

testing/marionette/element.js

@@ -738,7 +738,7 @@ element.toJson = function(obj, seenEls) {
   // HTMLElement
   else if ("nodeType" in obj && obj.nodeType == 1) {
     let uuid = seenEls.add(obj);
-    return {[element.Key]: uuid, [element.LegacyKey]: uuid};
+    return element.makeWebElement(uuid);
   }
 
   // arbitrary objects + files