Introduce toHTMLMapElement

To avoid direct use of static_cast, this patch introduces toHTMLMapElement for code cleanup.

Review URL: https://chromiumcodereview.appspot.com/19191002

git-svn-id: svn://svn.chromium.org/blink/trunk@154228 bbb929c8-8fbe-4397-9dbb-9b2b20218538