[attribution reporting] Include cross app web feature in chrome://flags

This will allow testing app to web alongside the rest of Privacy
Sandbox when turning on the Privacy Sandbox chrome://flag

Bug: 1428651
Change-Id: I35da661dff06d013459569fb692e61ea60fce739
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4377620


Reviewed-by: Avi Drissman <avi@chromium.org>
Auto-Submit: John Delaney <johnidel@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1123197}