[lacros] Add documentation for use with xvfb

Add an example to show how to use the test runner with xvfb

Change-Id: I8ccf0750024e8ae4a8e68e8178faf38d9aa37974
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2368415
Auto-Submit: Yuke Liao <liaoyuke@chromium.org>
Commit-Queue: Sven Zheng <svenzheng@chromium.org>
Reviewed-by: Sven Zheng <svenzheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800673}

build/lacros/test_runner.py

@@ -34,6 +34,11 @@
   useful to reproduce test failures, the version corresponds to the commit
   position of commits on the master branch, and a list of prebuilt versions can
   be found at: gs://ash-chromium-on-linux-prebuilts/x86_64.
+
+  ./testing/xvfb.py ./build/lacros/test_runner.py test out/lacros/browser_tests
+
+  The above command starts ash-chrome with xvfb instead of an X11 window, and
+  it's useful when running tests without a display attached, such as sshing.
 """
 
 import argparse