Support <input type=image> for submission to a <dialog>

As described in
http://www.whatwg.org/specs/web-apps/current-work/multipage/association-of-controls-and-forms.html#submit-dialog

BUG=304827

Review URL: https://codereview.chromium.org/31823002

git-svn-id: svn://svn.chromium.org/blink/trunk@160175 bbb929c8-8fbe-4397-9dbb-9b2b20218538