Value set in form submission depends on return value of onSubmit

OnSubmit is cancellable i.e. if return false or defaultPrevented, it should abort form submission.

Allows cancelling or proceeding depending onSubmit value as well handles form submission from JS.

R=tkent, keishi1
BUG=196640
TEST=Submitted values differs on return type of onsubmit. If true, changed values are sent. If false it aborts form submission.

Review URL: https://codereview.chromium.org/177683004

git-svn-id: svn://svn.chromium.org/blink/trunk@169185 bbb929c8-8fbe-4397-9dbb-9b2b20218538