added an iframe onload so that the update postMessage can't get lost.
added an iframe onload so that the update postMessage can't get lost.

humphd/thimble.mozilla.org,mozilla/thimble.mozilla.org,humphd/thimble.mozilla.org,mozilla/thimble.mozilla.org,humphd/thimble.mozilla.org,mozilla/thimble.mozilla.org,humphd/thimble.mozilla.org,mozilla/thimble.mozilla.org