Update is-valid flag upon resetting input elements.

If the value of an input element is reset back to empty, make sure we
sync its is-valid flag when doing so. If not, form control elements
marked as 'required' may end up being incorrectly labelled as valid.

R=
BUG=336818

Review URL: https://codereview.chromium.org/148463008

git-svn-id: svn://svn.chromium.org/blink/trunk@165942 bbb929c8-8fbe-4397-9dbb-9b2b20218538