trace: use standard library context

Fixes golang/go#20056

Change-Id: I139d531ae87273fdc2409d29d63bb9d67680f30a
Reviewed-on: https://go-review.googlesource.com/41290
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
1 file changed