Flaky Test: http/wpt/css/css-animations/start-animation-001.html

Flaky Test: http/wpt/css/css-animations/start-animation-001.html
https://bugs.webkit.org/show_bug.cgi?id=190903

This patch modifies start-animation-001.html to rely on the square positions returned by the
Web Process in order to know when they are supposed to be rendered behind the covering rect.
This should make it more robust while still testing the original bug 186129.

Patch by Frederic Wang <fwang@igalia.com> on 2019-01-04
Reviewed by Antonio Gomes.

* http/wpt/css/css-animations/start-animation-001.html:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@239607 268f45cc-cd09-0410-ab3c-d52691b4dbfc