[IncognitoWindowing] Creates flag IncognitoThemeOverlayTesting

Creates a feature flag to enable the new incognito theme
overlay in the current window. This will be used for testing the
incognito theme overlay.

Bug: 435020430
Change-Id: If4dd071a58d071a60c65e0070605fc216fec595b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6825292
Commit-Queue: Joel Tan-Aristy <jtanaristy@google.com>
Reviewed-by: Shu Yang <shuyng@google.com>
Reviewed-by: Wenyu Fu <wenyufu@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1500456}