Remove Internals::numberOfActiveAnimations

The Internals interface numberOfActiveAnimations is no longer needed
as we now have AnimationTimeline::getAnimationPlayers.

I've tweaked the values in the webkit-clip-path-equality.html test as
currently it uses 0px for the center values but cssText returns this
as 0%.

Review URL: https://codereview.chromium.org/399843006

git-svn-id: svn://svn.chromium.org/blink/trunk@178609 bbb929c8-8fbe-4397-9dbb-9b2b20218538