1. Add a new preference, layout.css.step-position-jump.enabled, for
   step(_, jump-*) timing functions.
2. We still keep JumpEnd and End tags, even though there is no difference
   between them. Therefore, we could disable the preference if needed.
3. Update the calculation of StepTiming to match the algorithm in the spec.
4. For servo, we implement the correct step function algorithm except
   for the handling of before_flag. This could be fixed later.

Depends on D9313

Differential Revision: https://phabricator.services.mozilla.com/D9314