forms/Autofill: fix unsigned underflow error in <select>

Fix unsigned underflow error and prevent change events from being dispatched
when the selected index of a <select> hasn't actually changed.

This broke AutofillInteractiveTest.OnChangeAfterAutofill on the chrome side,
which will be re-enabled here: https://codereview.chromium.org/214823011/

BUG=353691
R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/217583002

git-svn-id: svn://svn.chromium.org/blink/trunk@170393 bbb929c8-8fbe-4397-9dbb-9b2b20218538