If the assert contained '%' symbols, it would be interpreted as a
substituion.

BUG=angleproject:1189

Change-Id: Ib347bc1a892397373e7318e032070c0337f110de
Reviewed-on: https://chromium-review.googlesource.com/316410
Tryjob-Request: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>