Crash fix, submitting dialog forms.

The crash occurs when a form has method=dialog but no submit button and is then
submitted (e.g., via JavaScript, myform.submit()). This code path is only hit
when the experimental runtime feature "dialogElementEnabled" is enabled (e.g.,
content_shell does that).

BUG=346147
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/177093024

git-svn-id: svn://svn.chromium.org/blink/trunk@167910 bbb929c8-8fbe-4397-9dbb-9b2b20218538