It is overwritten by ConfigSet::add.

BUG=angleproject:892

Change-Id: I23becb235969c6aeda216014fd06880bd7cc5f67
Reviewed-on: https://chromium-review.googlesource.com/269410Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>