Add the HTMLIFrameElement.allowFullscreen reflected attribute

http://whatwg.org/html#dom-iframe-allowfullscreen

BUG=383813

Review URL: https://codereview.chromium.org/398843002

git-svn-id: svn://svn.chromium.org/blink/trunk@178555 bbb929c8-8fbe-4397-9dbb-9b2b20218538