Adds flag to enable Private State Tokens Dev UI

Change-Id: I335c73b513fcc7179b69c9bb463e69cdd5945f8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5582590


Reviewed-by: Fiona Macintosh <fmacintosh@google.com>
Reviewed-by: Dylan Cutler <dylancutler@google.com>
Commit-Queue: Aimen Moten <aimenmoten@google.com>
Cr-Commit-Position: refs/heads/main@{#1313025}