Reland: Only report a warning instead of an error.

BUG=angleproject:989
BUG=483252

Change-Id: Ife3e7759cdef6bc0f41cae3c58c307682b608279
Reviewed-on: https://chromium-review.googlesource.com/269404Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>