Merges updates listed in
https://github.com/google/pprof/compare/0e0e5b725...a74ae6f

Update #24443

cmd/vendor/vendor.json was updated manually.

Change-Id: I15d5fe82ac18263d4d54f5773cee0e197e93dd59
Reviewed-on: https://go-review.googlesource.com/101736Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>