GitLab

See CL 40291. ctx.Err() is defined to only return non-nil exactly
when ctx.Done() returns a closed channel.

Change-Id: I12f51d8c42228f759273319b3ccc28012cb9fc73
Reviewed-on: https://go-review.googlesource.com/71310


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>