Improve async approach of form-method-dialog.html.

form-method-dialog.html had flaky timeouts and failures.  As described
in the bug, the flakiness may be caused by directly moving on to the
next testcase in the event handler for the current testcase. This
modifies the test to queue up the next testcase via setTimeout so it's
not entered until the current testcase finishes completely.

BUG=310330

Review URL: https://codereview.chromium.org/29403004

git-svn-id: svn://svn.chromium.org/blink/trunk@160332 bbb929c8-8fbe-4397-9dbb-9b2b20218538