Use isHTMLTextAreaElement and toHTMLTextAreaElement

To avoid direct use of static_cast, this patch uses isHTMLHtmlElement and toHTMLHtmlElement for code cleanup.

Review URL: https://chromiumcodereview.appspot.com/19096009

git-svn-id: svn://svn.chromium.org/blink/trunk@154118 bbb929c8-8fbe-4397-9dbb-9b2b20218538