Web Animations CSS: Add missing return in calculateAnimationCompositableValues

The common case where we have no new or cancelled animations is supposed
to exit early but I missed a return statement in crrev.com/52093002.

BUG=258896

Review URL: https://codereview.chromium.org/72513002

git-svn-id: svn://svn.chromium.org/blink/trunk@162071 bbb929c8-8fbe-4397-9dbb-9b2b20218538