Enable LoginExistingUsers test for mash

Suspect http://crrev.com/c/1259296 fixed it.

Bug: 888188
Change-Id: I40d9246fa5ad777cb91fd5aa9e740c1c550643bf
Reviewed-on: https://chromium-review.googlesource.com/c/1327552Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606635}

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

@@ -29,10 +29,6 @@
 -SelectToSpeak*
 -SwitchAccess*
 
-# Flaky. https://crbug.com/888188.
--ChromeSessionManagerTest.LoginExistingUsers
--ChromeSessionManagerTest.PRE_LoginExistingUsers
-
 # Failing on ASAN bot. crbug.com/882631
 -ExtensionWebRequestApiTest.WebRequestTypes