Revert 162182 "Web Animations CSS: Fix DocumentTimeline::numberO..."

Broke animation-controller-drt-api.html on most platforms.

> 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

TBR=steveblock@chromium.org

Review URL: https://codereview.chromium.org/69813018

git-svn-id: svn://svn.chromium.org/blink/trunk@162217 bbb929c8-8fbe-4397-9dbb-9b2b20218538