Web Animations CSS: Hook up CSS animation play state

The modification to Player's pause logic is a temporary work-around to allow us
to pause a Player for testing, without it impacting the CSS pause logic.

Note that when the Web Animations model is exposed to JavaScript, this will
include Players and Animations generated by CSS animations. We will then have
to support such animations being manipulated from both JavaScript and CSS, and
the solution for this will avoid the need for this work-around.

BUG=258896
R=dstockwell

Review URL: https://codereview.chromium.org/38673003

git-svn-id: svn://svn.chromium.org/blink/trunk@160417 bbb929c8-8fbe-4397-9dbb-9b2b20218538