Animations: Remove SortInfo from AnimationPlayer.

SortInfo object is redundant in AnimationPlayer. Instead, the sequence number can be used instead.

BUG=407474

Review URL: https://codereview.chromium.org/493013004

git-svn-id: svn://svn.chromium.org/blink/trunk@180893 bbb929c8-8fbe-4397-9dbb-9b2b20218538