HTMLOptionsCollection is always rooted at a HTMLSelectElement.
<https://webkit.org/b/123719>

Tighten up HTMLOptionsCollection by making the constructor take
a HTMLSelectElement& and adding a HTMLSelectElement& getter instead
of casting all over the place. Removed now-pointless assertions.

Reviewed by Sam Weinig.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@158557 268f45cc-cd09-0410-ab3c-d52691b4dbfc