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The stub account on ChromeOS is treated specially for test purposes.
Remove this special-casing since we have unified the interface for
setting the primary account on ChromeOS with that on other platforms.

Update tests: remove expectations that "the stub user is signed in by
default on browsertests".

Bug: 1014074
Change-Id: I37a812e060631c101d1b2d7d897c785c4c200f47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872066
Commit-Queue: Anastasiia Nikolaienko <anastasiian@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Kush Sinha <sinhak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#717684}