Add focus flag infrastructure and core utilities

Introduce the foundational components for the --focus command-line flag
feature in //ui/startup/focus/. This change adds the core utilities,
data structures, and helper classes needed to support tab/window
focusing functionality without integrating them into the main startup
flow.

Bug: 439499872
Change-Id: I60f402935023772496a4e2282d95d1aa49e8dd6a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6850334
Reviewed-by: Erik Chen <erikchen@chromium.org>
Reviewed-by: Kaan Alsan <alsan@chromium.org>
Commit-Queue: Helmut Januschka <helmut@januschka.com>
Cr-Commit-Position: refs/heads/main@{#1520059}