Also improve debug messages in window_test.

BUG=angleproject:1273

Change-Id: Ie47dc1d7ad3be5a55d2a199334bba606f2b45ad0
Reviewed-on: https://chromium-review.googlesource.com/322350
Tryjob-Request: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>