Fix race in fast/loader/onunload-form-submit-crash.html.

The setTimeout was sometimes firing before the form was parsed,
so document.myForm would be null.

BUG=278518
R=rjkroege@chromium.org

Review URL: https://codereview.chromium.org/23406003

git-svn-id: svn://svn.chromium.org/blink/trunk@156656 bbb929c8-8fbe-4397-9dbb-9b2b20218538