Web Animations CSS: Fix DocumentTimeline::numberOfActiveAnimationsForTesting()

This is used by internals.numberOfActiveAnimations() in DRT and is part of the
reason for the failure in repeated-firing-background-color.html

Also beef up animation-controller-drt-api.html to test this fix.

R=rjwright

Review URL: https://codereview.chromium.org/66953009

git-svn-id: svn://svn.chromium.org/blink/trunk@162182 bbb929c8-8fbe-4397-9dbb-9b2b20218538