Allow form submission for forms with "hidden" for submit with enter key 

Currently the forms are not submitted if the Submit button does not have
a renderer. This patch removes that restriction. This also matches the behavior with Firefox & IE

BUG=89586

Review URL: https://codereview.chromium.org/171603007

git-svn-id: svn://svn.chromium.org/blink/trunk@167857 bbb929c8-8fbe-4397-9dbb-9b2b20218538