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This change makes UpgradeDetector an observer of the process's
BuildState, removing its own detection. It instead creates an instance
of either an InstalledVersionPoller (for desktop Chrome) or an
InstalledVersionUpdater (for Chrome OS) to do the actual detection work.

BUG=1043624

Change-Id: Ib04b0f588a3d035f2ce0300561ed86d9024c7b3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2017352
Commit-Queue: Greg Thompson <grt@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738381}