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Add unit test for CrostiniPackageService. Also fixes a corner case found
by unit test (queuing uninstalls while install starts up).

BUG=822514
TEST=Ran unit test

Change-Id: Ieede7119febda6a2fb67ca9835e8d859ddf08400
Reviewed-on: https://chromium-review.googlesource.com/c/1401611
Commit-Queue: Ian Barkley-Yeung <iby@chromium.org>
Reviewed-by: Timothy Loh <timloh@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630485}