Add scrolling attribute for <iframe>

[thomasboyt]

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. 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.

[zpao]

Based on that bug, this will likely not be fixed in blink for a couple years :/

MUST_USE_PROPERTY and MUST_USE_ATTRIBUTE effect how changes are applied. For properties, we just do node.scrolling = value, for attributes we use `node.setAttribute('scrolling', value). When unspecified it means both should work (because the DOM is super consistent, lol) and we'll use whichever we think is faster. Currently that's property access. We don't have automated tests for these yet, so the best thing to do is actually use the component and property (I usually modify an example), and make sure when you change the value in render, the DOM is correctly changed.

[zpao]

Also, can you sign the CLA? https://code.facebook.com/cla

[thomasboyt]

Signed!

[zpao]

Pulling in now. I confirmed that both node.scrolling = value and node.setAttribute('scrolling', value) work so there's no need to specify.

[zpao]

Thanks!