Reland: Only report a warning instead of an error.

BUG=angleproject:989
BUG=483252

Change-Id: Ibf9adbf423cd9dee20ec45b8d2ea42bcfd9311be
Reviewed-on: https://chromium-review.googlesource.com/269205Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>