Web Animations: Don't compare start time for AnimationPlayer sorting

As per recent spec changes, the start time should no longer factor in to
AnimationPlayer sort order. I've left in the SortInfo class instead of
simply holding on to an unsigned sequence number since we may in the
future add a mechanism for changing sort order, although this is easy
to remove later if won't be adding such functionality.

Review URL: https://codereview.chromium.org/399033002

git-svn-id: svn://svn.chromium.org/blink/trunk@178531 bbb929c8-8fbe-4397-9dbb-9b2b20218538