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There have been recent crashes while trying to launch apps when these
profile types make their way through various APIs.

Although this test file doesn't deal with _launching_ the tasks, there
are planned changes to where file handlers may be stored, and we don't
want these expectations to change due to the profile type that is
passed to the file manager, and other APIs.

This CL introduces a "mixin" to allow any InProcessBrowserTest harness
to set up a command line with the de-facto settings used to run
browser tests under incognito and guest profiles.

Bug: 1122379
Change-Id: I16e8099be24043427a1ecc4b73d6fae8422f0e71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377041Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802167}