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This CL adds some tests for the linux package install flow in the File
Manager. It also fixes a bug in the CrostiniManager where package
progress observers were not correctly removed, as discovered by these
tests.

Bug: 868221
Change-Id: I2b6adf2530ff43a0b22fe2b552bb877f98fd61c7
Reviewed-on: https://chromium-review.googlesource.com/1152643Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Timothy Loh <timloh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578975}