iwa: Remove kIsolatedWebApps entry from about://flags on ChromeOS

The feature has been default-on since M128. This change also removes
the flag from selected ash test files and base test classes on CrOS.

Bug: 330854968
Change-Id: I25a5bf9904abf1f0ad1bf8c34b7983d839f1571c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6286249
Reviewed-by: Sergii Bykov <sbykov@google.com>
Reviewed-by: Jeremie Boulic <jboulic@chromium.org>
Auto-Submit: Andrew Rayskiy <greengrape@google.com>
Commit-Queue: Jeremie Boulic <jboulic@chromium.org>
Reviewed-by: Simon Hangl <simonha@google.com>
Cr-Commit-Position: refs/heads/main@{#1422855}