Make window-onerror-10.html not flaky

This newly-added test triggers exceptions in an onload handler and a setTimeout,
but the ordering of those two was not consistent.

Now the setTimeout is called from the onload handler, ensuring that one always
runs before the other.

TBR=mkwst@chromium.org

Review URL: https://codereview.chromium.org/20377003

git-svn-id: svn://svn.chromium.org/blink/trunk@154922 bbb929c8-8fbe-4397-9dbb-9b2b20218538