Move isAutofilled and setAutofilled from HTMLInputElement to HTMLFormControlElement

This is in support of enabling Autofill for <textarea> elements.

BUG=280499
TEST=fast/forms/*autofill*
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/26622007

git-svn-id: svn://svn.chromium.org/blink/trunk@159316 bbb929c8-8fbe-4397-9dbb-9b2b20218538