Web Animations: Add NumberOfActiveAnimationsForTesting

Add call to determine number of active animations.

BUG=271815

Review URL: https://chromiumcodereview.appspot.com/22403006

git-svn-id: svn://svn.chromium.org/blink/trunk@155990 bbb929c8-8fbe-4397-9dbb-9b2b20218538