Introduce toHTMLSelectElement and remove isHTMLSelectElement

To avoid direct use of static_cast, this patch introduces toHTMLSelectElement for code cleanup.
Additionally, this patch removes isHTMLSelectElement because not all element subclasses can be checked by a combination of tag names.

Review URL: https://chromiumcodereview.appspot.com/19488002

git-svn-id: svn://svn.chromium.org/blink/trunk@154365 bbb929c8-8fbe-4397-9dbb-9b2b20218538