CSS Animations: Generate a single player for each keyframe animation.

Logic to split keyframe animations into multiple players was required before
Web Animations supported per-keyframe easings.

CSS Animations were updated to use per-keyframe easings in r169119[1], but this
logic remained.

[1] https://src.chromium.org/viewvc/blink?view=rev&revision=169119

BUG=346590

Review URL: https://codereview.chromium.org/214603002

git-svn-id: svn://svn.chromium.org/blink/trunk@170302 bbb929c8-8fbe-4397-9dbb-9b2b20218538