Enable early process singleton on Windows/BETA

This experiment is going on for multiple months on Canary/Dev.
We intend to enable by default soon. As a step forward, we
are rolling out to 50% BETA.

Affected platform: Windows only.

Bug: 1340599
Change-Id: Idd457115782bcfd579d49fad9108be5359558c5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4544613


Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Luc Nguyen <lucnguyen@google.com>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147894}