Disable flaky chrome session manager login tests

Bug: 888188
Change-Id: I9056c918ceda50d5c50d7dfbadef5c22f7e80d34
Reviewed-on: https://chromium-review.googlesource.com/1246948Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Ryan Cui <rcui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594842}

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

@@ -162,6 +162,10 @@
 # Ash native views tests.
 -ChromeNativeAppWindowViewsAuraAshBrowserTest.*
 
+# Flaky.  crbug.com/888188.
+-ChromeSessionManagerTest.LoginExistingUsers
+-ChromeSessionManagerTest.PRE_LoginExistingUsers
+
 # Context menu tests.
 -ContextMenuBrowserTest.ContextMenuEntriesAreDisabledInLockedFullscreen