Add scrolling attribute for <iframe>

The `scrolling` attribute was dropped from HTML5 in favor of just setting the `overflow` property, but unfortunately [Chrome doesn't support `overflow` on iframes yet](https://code.google.com/p/chromium/issues/detail?id=169222). This adds `scrolling` to the list of supported DOM properties so that it can be used when rendering an iframe from React.

I'm not really sure how `MUST_USE_PROPERTY` or `MUST_USE_ATTRIBUTE` work, will be happy to set this to one or the other if needed.