De-flake some animation tests

De-flake some animation tests
https://bugs.webkit.org/show_bug.cgi?id=188356

Reviewed by Dean Jackson.

The tests in legacy-animation-engine were flakey because the animation can start before
we get a chance to register the event listener. Fix by starting the animation after the
load event fires. Make the non-legacy tests match.

* compositing/visible-rect/animated-from-none.html:
* compositing/visible-rect/animated.html:
* legacy-animation-engine/compositing/visible-rect/animated-from-none.html:
* legacy-animation-engine/compositing/visible-rect/animated.html:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@234641 268f45cc-cd09-0410-ab3c-d52691b4dbfc