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As site isolation is now enabled by default, the only remaining effect
of the SitePerProcess user policy is to prevent the user from disabling
site isolation.

Instead of passing --site-per-process through a chrome process restart
to enforce this, simply avoid passing swithces::kDisableSiteIsolation
when the policy is set to true.

Bug: 964068
Test: browser_test --gtest_filter=*SiteIsolationFlagHandling*
Change-Id: I52b89604932a2f41e94ccb3ede7e48ec591d73f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1653948
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671565}