[lacros] Improve lacros testing documentation

This CL adds an example to show how to use tests against a locally
built ash-chrome.

Change-Id: I7f7cc04a3ae594fb388c9b81a13ac5375d72db35
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2476902Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817671}

docs/lacros.md

@@ -67,6 +67,12 @@ mojo crosapi connection. This requires the test target
 
 `./build/lacros/test_runner.py test out/lacros/lacros_chrome_browsertests --gtest_filter=ScreenManagerLacrosBrowserTest.*`
 
+By default, the test runner downloads a prebuilt ash-chrome, add the
+`--ash-chrome-path` command line argument to run the test against a locally
+built version of Ash:
+
+`./build/lacros/test_runner.py test --ash-chrome-path=out/ash/chrome out/lacros/lacros_chrome_browsertests --gtest_filter=ScreenManagerLacrosBrowserTest.*`
+
 If you're sshing to your desktop, please prefix the command with
 `./testing/xvfb.py`.