Enable EarlyProcessSingleton on Linux Canary/Dev/Beta

This CL is adding the code to get an machine-id (when available) on Linux. The machine-id is used to split the population within the
different arms of the EarlyProcessSingleton experiments.

If the /etc/machine-id file is not present, the users will be part
of the "default" group.

Bug: 1340599
Change-Id: Ia15292987d5346b773a98109279d5bda5c038d0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4563733


Reviewed-by: Luc Nguyen <lucnguyen@google.com>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1149374}