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At the moment the Contained Shell launches any time its feature flag is
enabled. But since we want users to toggle the experience from settings,
we added a pref to control it. This change actually enforces this new
pref by aways checking it when we try to launch the experience.

Bug: 922687
Change-Id: Icf1ffa2422bbb68fda80e22f626d48bcbe8c8c18
Reviewed-on: https://chromium-review.googlesource.com/c/1477821
Commit-Queue: Lucas Tenório <ltenorio@chromium.org>
Reviewed-by: Mitsuru Oshima (OOO til 3/4) <oshima@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636447}