The recent vendored pprof update broke the iOS builders. The issue was
reported and patched upstream. Re-vendor the internal pprof copy.

Updates vendored pprof to commit 9e20b5b106e946f4cd1df94c1f6fe3f88456628d
from github.com/google/pprof (2017-11-08).

Fixes #22612

Change-Id: I74c46c75e92ce401e605c55e27d8545c0d66082c
Reviewed-on: https://go-review.googlesource.com/76651Reviewed-by: Elias Naur <elias.naur@gmail.com>