[css-viewport] CSS zoom should affect clip-path: path() strings.

[emilio]

Right now per spec it doesn't, but that doesn't match implementations, and it makes sense for it to affect it.

We have tests for this (css/css-masking/clip-path/clip-path-path-with-zoom.html etc), so I think just a tweak to the spec should be enough, wdyt @chrishtr?