Web Animations: Sort Players in DocumentTimeline::serviceAnimations

This patch makes Animations get serviced in the order specified by the
spec[1]. This means that once custom effect are implemented, they will
be queued up in the correct order. This also gets us closer to having
the animation stack in a correct order (I'm thinking of emptying the
stack after getting out the compositable values, as we traverse the
animations in the correct order).

[1] http://dev.w3.org/fxtf/web-animations/#the-animation-stack

BUG=334936

Review URL: https://codereview.chromium.org/185593010

git-svn-id: svn://svn.chromium.org/blink/trunk@168597 bbb929c8-8fbe-4397-9dbb-9b2b20218538