Deflake fast/loader/resource-request-callbacks.html.

Background images aren't guaranteed to start loading after
a setTimeout(0). Need to force a style recalc.

Also, update the test to remove the setTimeout(100). I don't
see what this was needed for and it makes the test slow.
And updated the number of expected resource requests. This
dropped when we removed js-test-post.js.

BUG=373072

Review URL: https://codereview.chromium.org/294473003

git-svn-id: svn://svn.chromium.org/blink/trunk@174303 bbb929c8-8fbe-4397-9dbb-9b2b20218538