Add a WebTextAreaElement wrapper around HTMLTextAreaElement.

Adding this wrapper is the first step towards enabling Autofill for <textarea> elements.

BUG=280499
TEST=none
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/25842002

git-svn-id: svn://svn.chromium.org/blink/trunk@158901 bbb929c8-8fbe-4397-9dbb-9b2b20218538