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This CL introduces a new model for the state of the build and
controllers to drive it for desktop Chrome and Chrome OS. In its
introduction here, the model holds whether or not an update is
available, its version, the "critical" version (if set), and whether or
not the new version is a rollback.

This CL does not activate the new model or controllers in the browser. A
subsequent CL will give the browser a process-wide BuildState instance,
a single controller to update the BuildState when an update is
available, and make UpgradeDetector an observer of the process's
BuildState.

BUG=1043624

Change-Id: I50be6b313ee152f2351e823fbd8637aac0c5c52a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2017520
Commit-Queue: Greg Thompson <grt@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737757}