Use tighter typing in RadioNodeList::elementMatches()

Use tighter typing in RadioNodeList::elementMatches() to give C++11 compilers
a chance to devirtualize the call to HTMLImageElement::formOwner().

R=adamk@chromium.org

Review URL: https://codereview.chromium.org/475613003

git-svn-id: svn://svn.chromium.org/blink/trunk@180369 bbb929c8-8fbe-4397-9dbb-9b2b20218538