Web Animations: Add Player::play() and replace setPaused() with unpause()

This adds play() to the Player class as per the expected idl interface
(not yet exposed in this patch though), and replaces setPaused with
unpause, only for use internally and within CSS.

BUG=334926

Review URL: https://codereview.chromium.org/148873005

git-svn-id: svn://svn.chromium.org/blink/trunk@166248 bbb929c8-8fbe-4397-9dbb-9b2b20218538