Web Animations: Add documentTimelineZero

Implement document zero time & set to performance.timing.domInteractive.

BUG=258896

Review URL: https://chromiumcodereview.appspot.com/22391009

git-svn-id: svn://svn.chromium.org/blink/trunk@156077 bbb929c8-8fbe-4397-9dbb-9b2b20218538