Change url.baseline.snapshot -> url.baseline

In deeb7d0587281c9b we changed how the concept of baseline works. Now,
Url#baseline returns a Snapshot instead of a Baseline object, so we no
longer need to call #snapshot on it. This fixes an error that happened
when creating snapshots that were diffed against a baseline.

Change-Id: I4e0ea80b5dbda52e903753f1adac566c96327e0e