chromeos-mash: Fix flaky UserAddingScreenTest.AddingSeveralUsers

User adding screen does not wait for user switching animation
with Window Service (crbug.com/875111). But we are still observing
the animation and the screen is dismissed when the animation ends.
This could cause the user adding screen being dismissed unexpectedly
as shown in the single process mash browser tests. The CL fixes
the issue by not observing the animation in such case.

Bug: 891383
Change-Id: I21d8735d517f7cbd8a17c2902dd73d9f05e282cc
Reviewed-on: https://chromium-review.googlesource.com/c/1259296
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596344}

chrome/browser/chromeos/login/ui/login_display_host_webui.cc

@@ -590,13 +590,15 @@ void LoginDisplayHostWebUI::OnStartUserAdding() {
   else
     finalize_animation_type_ = ANIMATION_NONE;
 
-  // Observe the user switch animation and defer the deletion of itself only
-  // after the animation is finished.
-  MultiUserWindowManager* window_manager =
-      MultiUserWindowManager::GetInstance();
-  // MultiUserWindowManager instance might be nullptr in a unit test.
-  if (window_manager)
-    window_manager->AddObserver(this);
+  if (finalize_animation_type_ == ANIMATION_ADD_USER) {
+    // Observe the user switch animation and defer the deletion of itself only
+    // after the animation is finished.
+    MultiUserWindowManager* window_manager =
+        MultiUserWindowManager::GetInstance();
+    // MultiUserWindowManager instance might be nullptr in a unit test.
+    if (window_manager)
+      window_manager->AddObserver(this);
+  }
 
   VLOG(1) << "Login WebUI >> user adding";
   if (!login_window_)

testing/buildbot/filters/chromeos.single_process_mash.browser_tests.filter

@@ -232,10 +232,6 @@
 # Flaky tests. crbug.com/880584
 -UnifiedAutoplaySettingBrowserTest.*
 
-# This seems to timeout only on the msan bot, but I suspect it could equally
-# occur on the non-msan bots. https://crbug.com/891383
--UserAddingScreenTest.AddingSeveralUsers
-
 # Window Open API tests.  https://crbug.com/815379
 -WindowOpenApiTest.OpenLockedFullscreenWindow
 -WindowOpenApiTest.RemoveLockedFullscreenFromWindow