Web Animations CSS: Make AnimationClock nondecreasing and drop assertion

This patch modifies the case where we try and update the AnimationClock
with a time in the past with logic that won't update the clock's time.
This case would be hit in cases where one Document is already running
and getting its animations serviced and another Document is created.
The new Document would have its clock updated in its constructor at
the current time, but the timelines would get called with the time
at the start of the frame boundary, in the past. As we assume the
clock is nondecreasing (e.g. we may fire the same event multiple
times if it isn't), we now ensure this behaviour for the clock.

BUG=280439

Review URL: https://codereview.chromium.org/68853006

git-svn-id: svn://svn.chromium.org/blink/trunk@161747 bbb929c8-8fbe-4397-9dbb-9b2b20218538