Web Animations: Sort Animations in the AnimationStack

This patch sorts animations in the stack so that they will be applied in the
correct order. Note that due to the way compositor integration currently is
implemented, we don't know the exact time when new animations will be started,
but expect it will be very close to the current timeline time.

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

BUG=

Review URL: https://codereview.chromium.org/210703002

git-svn-id: svn://svn.chromium.org/blink/trunk@170257 bbb929c8-8fbe-4397-9dbb-9b2b20218538