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This CL fixes a crash happening after creating a signed in profile when
the policy is set.
This CL also adds browser tests checking that the signin profile is
appropriately marked and unmarked ephemeral, and correctly destroyed
at startup if required.

Fixed: 1164314
Change-Id: I078fbc127d42267261b305149aff10e71ac53b59
Cq-Do-Not-Cancel-Tryjobs: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2617926
Commit-Queue: David Roger <droger@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841973}