GitLab

This test was running with an average speed close to its timeout and
was occasionally exceeding it, causing flakes. Marking it as slow as
per wpt guidelines.

Bug: 1073460
Change-Id: I2028d64ed727242ec3873c5172a08ecebbc45a73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2163395
Commit-Queue: George Steel <gtsteel@chromium.org>
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762061}