Add a test for unthemed browser frames

Guard against accidental theming of the browser frame in ChromeOS.

Follow up for https://crrev.com/7b03f492c

Change-Id: I32fa174ebc350f65558a358342124eae1c16d0b4
Bug: b:282841014
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4538684


Reviewed-by: Emily Shack <emshack@chromium.org>
Commit-Queue: Sean Kau <skau@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147407}