Reland "Mac: Record via metrics the number of open FDs and the soft FD limit."

This is a reland of a72c20e443285e06e064e67ec754d4f60d4e6d22

Fixes ProcessMetricsTest.GetChildOpenFdCount to not close guarded
FDs on macOS 10.10 and 10.11.

Original change's description:
> Mac: Record via metrics the number of open FDs and the soft FD limit.
>
> The histograms already exist on Linux, CrOS, and Android, so this simply
> wires up the values for macOS.
>
> In addition, this enables the FD count column in the Task Manager, since
> the data are now readily available.
>
> Bug: 714614
> Change-Id: I05f8063e8d89d3c6cdc68062bf4e591d3af9559c
> Reviewed-on: https://chromium-review.googlesource.com/c/1423339
> Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Reviewed-by: Mark Mentovai <mark@chromium.org>
> Commit-Queue: Robert Sesek <rsesek@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#624881}

Tbr: thakis@chromium.org
Bug: 714614
Change-Id: Icc79aa88826e9079845f06ed6c3f52139713b404
Reviewed-on: https://chromium-review.googlesource.com/c/1430719
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625329}
12 files changed