Fix flaky profile window browsertest

Fix ProfileWindowBrowserTest.OpenBrowserWindowForProfileWithSigninRequired on Linux
The test is flaky due to race conditionin the test body.

Bug: 904397, 901430
Change-Id: I80b73f507e3826fc7a707379b89b39a3e7aa68c5
Reviewed-on: https://chromium-review.googlesource.com/c/1330032Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Owen Min <zmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607260}

chrome/browser/profiles/profile_window_browsertest.cc

@@ -247,10 +247,12 @@ IN_PROC_BROWSER_TEST_F(ProfileWindowBrowserTest,
                   .GetProfileAttributesWithPath(profile->GetPath(), &entry));
   entry->SetIsSigninRequired(true);
   size_t num_browsers = BrowserList::GetInstance()->size();
+  base::RunLoop run_loop;
+  UserManager::AddOnUserManagerShownCallbackForTesting(run_loop.QuitClosure());
   profiles::OpenBrowserWindowForProfile(
       ProfileManager::CreateCallback(), true, false, false, profile,
       Profile::CreateStatus::CREATE_STATUS_INITIALIZED);
-  base::RunLoop().RunUntilIdle();
+  run_loop.Run();
   EXPECT_EQ(num_browsers, BrowserList::GetInstance()->size());
   EXPECT_TRUE(UserManager::IsShowing());
 }