Refactor layout test xmlhttprequest-timeout.js

It's hard to understand what's being tested. Also some ok() calls can be
ignored since handleEvent calls testComplete() earlier.

Refactor and add comments.

BUG=none

Review URL: https://codereview.chromium.org/64363002

git-svn-id: svn://svn.chromium.org/blink/trunk@161768 bbb929c8-8fbe-4397-9dbb-9b2b20218538