Explicitly specify behavior when multiplying zero by infinity

There are a number of variables in the Web Animations model which may have a
value of infinity and which may be multiplied by zero. In IEEE 745,
multiplication of zero and infinity is undefined, but currently the spec does
not clarify what the desired result is.

In all cases, the correct result is zero and this change makes this explicit.
The following sections are updated.

- Calculating the active duration
  iteration duration is in [0, infinity]
  iteration count is in [0, infinity]

- Calculating the start offset
  iteration start is in [0, Infinity)
  iteration duration is in [0, infinity]

- Calculating the scaled active time
  playback rate is in (-infinity, infinity)
  active time (or active duration - active time) is in [0, Infinity]

- Calculating the transformed time
  scaled fraction is in [0, infinity)
  iteration duration is in [0, infinity]
Use an active time of negative zero when in the before phase and backwards fill

This allows us to specify that a step-at-start timing function has an output
time of zero when in this state, to match the behavior of all other timing
functions.