Set autofilled property to false when edit the autofilled text in textarea

R=isherman@chromium.org, tkent@chromium.org
BUG=399626

Review URL: https://codereview.chromium.org/426593011

git-svn-id: svn://svn.chromium.org/blink/trunk@179586 bbb929c8-8fbe-4397-9dbb-9b2b20218538