Web Animations: Set the DocumentTimeline's zero time when the Document reaches the 'loading' phase

It's possible for CSS animations to be created before the Document reaches the
'interactive' phase and we rely on the DocumentTimeline having a valid current
time when the animation is created,

Note that the new LayoutTest timeouts introduced here are due to a pre-existing
bug with the generation of events for CSS animations. It seems that with this
change we now hit this condition more frequently. A fix is coming soon.

BUG=240653
R=dstockwell

Review URL: https://chromiumcodereview.appspot.com/23327003

git-svn-id: svn://svn.chromium.org/blink/trunk@156458 bbb929c8-8fbe-4397-9dbb-9b2b20218538