I am currently working on a CL:https://chromium-review.googlesource.com/c/chromium/src/+/986889
to move PowerPrefs to ash, in that PowerPrefs's initialization depends on the initialization of chromeos::PowerPolicyController. chromeos::PowerPolicyController's initialization depends on chromeos::PowerManagerClient.

Before, those DBus clients are initialized by chrome (in chrome_browser_main_chromeos.cc) and only used by chrome before mustash.

Now I can see the plan for mash is to initialize DBus clients in window manager service for mash. In current mash browser test that requires injecting fake dbus clients, there could be problem.

(1) In browser test SetUpInProcessBrowserTestFixture(), DBusThreadManager initialized for testing could be done by chromeos::DBusThreadManager::GetSetterForTesting().
(2) Meanwhile, ash::WindowManagerService::OnStart() would enter here:
https://cs.chromium.org/chromium/src/ash/window_manager_service.cc?q=windowmanagerservice&dr=CSs&l=86, where it sees DBusThreadManager is not initialized (it sounds a racing to me) and initializes DBusThreadManager. Thus any injected fake dbus clients in browser tests are not injected. And any following ash code that depends on the dbus clients will not depend on the fake dbus clients in browser tests.