context: add lock in Cause to avoid race

context: add lock in Cause to avoid race

Change-Id: I8d970e8db859bdd17390cfbc22cc2ba0d326ed0c
Reviewed-on: https://go-review.googlesource.com/c/go/+/453735
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Sameer Ajmani <sameer@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>