Update lacros documentation for testing.

Change-Id: I19308526e68a7699f26e57d90219fb73cb279fe5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2444136
Commit-Queue: Erik Chen <erikchen@chromium.org>
Auto-Submit: Erik Chen <erikchen@chromium.org>
Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813314}

docs/lacros.md

@@ -57,11 +57,16 @@ Lacros bugs can be filed under component: OS>LaCrOs.
 ## Testing
 
 Most test suites require ash-chrome to be running in order to provide a basic
-Wayland server and the crosapi implementation. This requires a special test
-runner:
+Wayland server. This requires a special test runner:
 
 `./build/lacros/test_runner.py test out/lacros/browser_tests --gtest_filter=BrowserTest.Title`
 
+Some test suites require ash-chrome to provide both a Wayland server and a valid
+mojo crosapi connection. This requires the test target
+`lacros_chrome_browsertests`:
+
+`./build/lacros/test_runner.py test out/lacros/lacros_chrome_browsertests --gtest_filter=ScreenManagerLacrosBrowserTest.*`
+
 If you're sshing to your desktop, please prefix the command with
 `./testing/xvfb.py`.