Update argument name for Screenshot() in Kombucha tests

The current argument name of `baseline` is very ambiguous. `baseline_cl`
is much more descriptive and will not require readers to read the
Screenshot() documentation to understand what it should be.

Fixed: 331854827
Change-Id: I3cfe15cf9aa1f0f4f1f047572827dabe24263ece
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5405914


Reviewed-by: Dana Fried <dfried@chromium.org>
Commit-Queue: Vinny Persky <vinnypersky@google.com>
Cr-Commit-Position: refs/heads/main@{#1280249}