Fixes #12438
Updates #15229 (to decide context key variable naming convention)

Change-Id: I3ba423e91b689e232143247d044495a12c97a7d2
Reviewed-on: https://go-review.googlesource.com/21829Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>