Have HTMLFormElement's named getter return a RadioNodeList.

The getter for HTMLFormElement is now required to return a
RadioNodeList if multiple form elements match,

 http://www.whatwg.org/specs/web-apps/current-work/#dom-form-nameditem

Provide this, with the extra wrinkle of having to support a live
RadioNodeList over only <img> elements if no 'listed elements' match
the given ID/name.

R=esprehn@chromium.org,tkent@chromium.org
BUG=315851
TEST=fast/forms/form-radio-node-list
TEST=fast/forms/form-radio-img-node-list

Review URL: https://codereview.chromium.org/119063002

git-svn-id: svn://svn.chromium.org/blink/trunk@164232 bbb929c8-8fbe-4397-9dbb-9b2b20218538