Web Animations: Fix handing of m_player on TimedItem

Missed initialization and inappropriate assertion were causing flaky failures
on the mac_layout bots.

TBR=esprehn
BUG=232273

Review URL: https://chromiumcodereview.appspot.com/19888008

git-svn-id: svn://svn.chromium.org/blink/trunk@154810 bbb929c8-8fbe-4397-9dbb-9b2b20218538