Web Animations: Fix disassociating TimedItems from Players

This patch fixes the behaviour of setting the source of a player (either
by calling setSource or in the constructor) to properly disassociate the
source from any player it may previously be attached to.

BUG=334926

Review URL: https://codereview.chromium.org/171983002

git-svn-id: svn://svn.chromium.org/blink/trunk@167455 bbb929c8-8fbe-4397-9dbb-9b2b20218538