Make it obvious an HTMLOptionsCollection can only contain HTMLOptionElements

Make it obvious an HTMLOptionsCollection can only contain HTMLOptionElements by
using tighter typing. This CL also moves the Element matching code from
HTMLCollection to the HTMLOptionsCollection subclass for consistency with other
HTMLCollection subclasses.

R=adamk@chromium.org

Review URL: https://codereview.chromium.org/485093003

git-svn-id: svn://svn.chromium.org/blink/trunk@180676 bbb929c8-8fbe-4397-9dbb-9b2b20218538