This copies the golang.org/x/net/context package to the standard library.

It is imported from the x/net repo's git rev 1d9fd3b8333e (the most
recent modified to x/net/context as of 2016-03-07).

The corresponding change to x/net/context is in https://golang.org/cl/20347

Updates #14660

Change-Id: Ida14b1b7e115194d6218d9ac614548b9f41641cc
Reviewed-on: https://go-review.googlesource.com/20346Reviewed-by: Sameer Ajmani <sameer@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>