Web Animations: Tighten up an assertion in calculateScaledActiveTime()

The activeTime should always be less than or equal to the activeDuration, so
assert this in calculateScaledActiveTime().

Update unit tests to avoid this case.

R=dstockwell

Review URL: https://codereview.chromium.org/84943004

git-svn-id: svn://svn.chromium.org/blink/trunk@162604 bbb929c8-8fbe-4397-9dbb-9b2b20218538