Make HTMLOutputElement.htmlFor settable

Make HTMLOutputElement.htmlFor settable as per the specification:
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-button-element.html#the-output-element

Calling HTMLOutputElement.htmlFor="foo" should be equivalent to
HTMLOutputElement.value.htmlFor="foo". This behavior is consistent
with Firefox 23.

BUG=308034

Review URL: https://codereview.chromium.org/26693003

git-svn-id: svn://svn.chromium.org/blink/trunk@159838 bbb929c8-8fbe-4397-9dbb-9b2b20218538