Commit acc90c53 passed the trybots, lingered for weeks, and in the
meantime the type of this variable changed to a bool. I didn't rebase
and re-run the trybots before submitting.

Fixes #12832

Change-Id: If24fda227edd8207f8069c67f1c45f08e6ac215a
Reviewed-on: https://go-review.googlesource.com/15286Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>