arc: Avoid showing browser window in ARC++ Kiosk session

Chrome shouldn't be started for ARC++ Kiosk sessions.
However, if it's not started at all before "kSilentLaunch" check,
then bottom bar is shown.
Starting Chrome silently avoid it and shows only blank wallpaper until app is started.

As in future dedicated ARC++ Kiosk initialization flow with Splash Screen will be added,
the issue should pass away together with it, but we need temporary fix for M56 launch.

BUG=676185
TEST=Manual, Start ARC++ Kiosk sessions.

Review-Url: https://codereview.chromium.org/2604813002
Cr-Commit-Position: refs/heads/master@{#441933}

chrome/browser/chromeos/login/users/chrome_user_manager_impl.cc

@@ -889,6 +889,7 @@ void ChromeUserManagerImpl::ArcKioskAppLoggedIn(user_manager::User* user) {
   base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
   command_line->AppendSwitch(chromeos::switches::kEnableArc);
   command_line->AppendSwitch(::switches::kForceAndroidAppMode);
+  command_line->AppendSwitch(::switches::kSilentLaunch);
 
   // Disable window animation since kiosk app runs in a single full screen
   // window and window animation causes start-up janks.