Introduce toHTMLLinkElement

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

Review URL: https://chromiumcodereview.appspot.com/19186002

git-svn-id: svn://svn.chromium.org/blink/trunk@154224 bbb929c8-8fbe-4397-9dbb-9b2b20218538