cmd: update golang.org/x/tools to 3b9d20c52192

cmd: update golang.org/x/tools to 3b9d20c52192

To pick up CL 452155.

Done by
        go get -d golang.org/x/tools@master
        go mod tidy
        go mod vendor

Change-Id: I4dba4ba9f17123cfe567704b0d5e5fc16f9ffd83
Reviewed-on: https://go-review.googlesource.com/c/go/+/452615
Run-TryBot: Tim King <taking@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>