Clean up cros_ui.restart() API

cros_ui.restart() took a callable to allow a custom mechanism
for restarting the UI. The only thing using this was passing
in a pyauto method, which was deprecated ages ago. Remove this
but allow instead for the caller to specify that it's OK
for the UI to be down already when calling cros_ui.restart().

BUG=None
TEST=stop ui and then run login_OwnershipRetaken

Change-Id: I78286a5edc777f6a96e2b48e13d386c3a9fd6d0b
Reviewed-on: https://chromium-review.googlesource.com/210054
Tested-by: Chris Masone <cmasone@chromium.org>
Reviewed-by: Richard Barnette <jrbarnette@chromium.org>
Commit-Queue: Chris Masone <cmasone@chromium.org>
diff --git a/client/cros/ownership.py b/client/cros/ownership.py
index d5a5258..cc4c41a 100644
--- a/client/cros/ownership.py
+++ b/client/cros/ownership.py
@@ -87,7 +87,7 @@
     The UI must be stopped while we do this, or the session_manager will
     write the policy and key files out again.
     """
-    cros_ui.stop()
+    cros_ui.stop(allow_fail=not cros_ui.is_up())
     clear_ownership_files_no_restart()
     cros_ui.start()