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This forces explicit dependencies for code relying on //chromeos/login.
Only code that is part of the Chrome process should have these deps.

For minor BUILD.gn changes:
TBR=rdevlin.cronin@chromium.org,hidehiko@chromium.org

Bug: 918682
Change-Id: Ic443725f1f6763e36a7214f569542822db94482c
Reviewed-on: https://chromium-review.googlesource.com/c/1394800
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619976}