Add optional trybot for linux-wpt-content-shell-fyi.

This adds an optional trybot for wptrunner with content_shell,
so we can more easily test changes to achieve parity with
run_web_tests.

Bug: 1295373
Change-Id: Id5d6511ebe10ce7eb5558e77555189aaa6cde3b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3866553


Commit-Queue: Thorben Troebst <troebst@google.com>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1041700}