Enable DisplayRotationDefaultTest under SingleProcessMash

It seems to pass now.

Bug: 831826
Change-Id: Idacf939fcc05a76fa14824ffa3f5f2cc4f68f984
Reviewed-on: https://chromium-review.googlesource.com/c/1297403Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602243}

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

@@ -48,10 +48,6 @@
 -PDFExtensionTest.*
 -PDFExtensionHitTestTest.*
 
-# ash::Shell::display_manager() to update displays.
-# http://crbug.com/831826
--PolicyDisplayRotationDefault/DisplayRotationDefaultTest.*
-
 # JS failure: hasAccessToCurrentWindow: FAIL (no message)
 -LockScreenNoteTakingTest.*