Use the Document AnimationClock in SMILTimeContainer

It's trivial to replace the local AnimationClock in SMILTimeContainer
with the one from Document after making the observation that the only
current manipulations (updateTime/unfreeze) are already performed by
PageAnimator (DocumentAnimations::updateAnimationTimingForAnimationFrame)
and RenderLayerCompositor (DocumentAnimations::startPendingAnimations).

With this change it become possible to push the DiscardScope into
updateAnimationsAndScheduleFrameIfNeeded - so do that to simplify the
code (by reducing the number of places that instantiate a DiscardScope.)

BUG=231576

Review URL: https://codereview.chromium.org/198723005

git-svn-id: svn://svn.chromium.org/blink/trunk@169761 bbb929c8-8fbe-4397-9dbb-9b2b20218538