This change was originally done by Mike Kruskal in Google-internal cl/520045664.
(Undeclared extensions are no longer accepted Google-internally.)
Change-Id: I24d24ff2c7dc85920bd5dbe86724186d98783a18
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/609555
Reviewed-by: Christian Höppner <hoeppi@google.com>
Auto-Submit: Michael Stapelberg <stapelberg@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>