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This CL makes the sign-in tests more resilient:
* it clears the sign-in state on setup
* it commits pref changes on tear down to make sure that clearing the
last sign-in account is actually written to disk.

Bug: 961728

Change-Id: I833f3c888312c4a38ed13fa6bd9373989cf6dbb4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1605914
Commit-Queue: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659037}