Web Animations: Fix elpasedTime in animation events

This addresses the FIXME in CSSAnimations::EventDelegate::onEventCondition()
to pass the TimedItem to the event callback.

Also tighten up a test such that it would now detect the FIXME.

BUG=240653
R=dstockwell

Review URL: https://chromiumcodereview.appspot.com/23039013

git-svn-id: svn://svn.chromium.org/blink/trunk@156768 bbb929c8-8fbe-4397-9dbb-9b2b20218538