chromeos: Enable WebviewLoginTest in mash_browser_tests

The underlying bug was fixed and it's stable on the FYI bot.

Bug: 781391
Change-Id: I291f5b1ec376c27801c03c6217de65b9b98fd664
Reviewed-on: https://chromium-review.googlesource.com/772873
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517160}

testing/buildbot/filters/mash.browser_tests.filter

@@ -15,8 +15,6 @@ CrashRestoreComplexTest.*
 CrashRestoreSimpleTest.*
 DriveIntegrationServiceBrowserTest.*
 EnterpriseEnrollmentTest.*
-# Works, but flaky CHECK in TransportIdForWindow(). http://crbug.com/781391
-# EulaTest.*
 KioskAppManagerTest.*
 KioskAppUpdateServiceTest.*
 LoginPromptBrowserTest.*
@@ -32,8 +30,7 @@ ShutdownPolicyLockerTest.*
 UpdateScreenTest.*
 UserImageManagerTest.*
 UserSelectionScreenTest.*
-# Works, but flaky CHECK in TransportIdForWindow(). http://crbug.com/781391
-# WebviewLoginTest.*
+WebviewLoginTest.*
 
 # Tests from //chrome/browser/ui/ash.
 AcceleratorCommandsFullscreenBrowserTest.*