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The implementations are |NOTREACHED()| since users cannnot sign out of
Chrome in Chrome OS.

Bug: 820046
Change-Id: Ie559d52388418d5a8a182afdb8a08f92b726c21b
Reviewed-on: https://chromium-review.googlesource.com/1076291Reviewed-by: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563076}