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Some minor changes for cleaning up some of the installer code and
making it more testable. Unit tests being introduced here CL:2392163
and browser tests here CL:2392285.

Tests: Tested manually on my test device and ran the new tests.
Bug: b:161650651

Change-Id: Ia241ec790096b33b56b54fcb579b08eef3a9ace8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391891
Commit-Queue: Daniel Ng <danielng@google.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Nic Hollingum <hollingum@google.com>
Cr-Commit-Position: refs/heads/master@{#804857}