[css-transitions] Normalization of steps()

Hi!

In Chrome, Edge and probably Safari, a specified style of
transition-timing-function: steps(3)
normalizes to a computed value of
transition-timing-function: steps(3, end)

In Firefox, it doesn't. Timing function strings are always maintained
unaltered.

We should specify something, then conform to it! I don't have a strong
opinion what, but my irony muscle is twitching slightly because Edge seems
to do the opposite here to what they do with calc :)

Cheers,
    -Shane