Fix getIframe test helper

Chrome seems to always be ready on the first iteration, so this doesn't affect
Chrome, but on Firefox, the `iframe` is undefined in the recursive call.

This makes most of the test in `RunLayoutTests.html` pass in Firefox. The only
failing test is the one checking font sizes.

src/Layout-test-utils.js

@@ -91,7 +91,7 @@ var layoutTestUtils = (function() {
       _cachedIframe = iframe;
       return iframe;
     } else {
-      setTimeout(getIframe, 0);
+      setTimeout(getIframe.bind(null, iframe), 0);
     }
   }