Update vendored pprof to commit 4fc39a00b6b8c1aad05260f01429ec70e127252c
from github.com/google/pprof (2017-11-01).

Fixes #19380
Updates #21047

Change-Id: Ib64a94a45209039e5945acbcfa0392790c8ee41e
Reviewed-on: https://go-review.googlesource.com/57370
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>