Fixes a glitch on OSX where the root window would swap twice, causing a
glitch.

Change-Id: I5886105fb18ed8f45e69cd10b5d1cac49b82093f
Reviewed-on: https://chromium-review.googlesource.com/320291
Tryjob-Request: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>