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This CL adds the handling for non critical updates (warning time
greater than to zero) for the policy MinimumChromeVersionEnforced.
A timer is started based on the current warning time which triggers
when the deadline is reached.
To persist the deadline across reboots, 2 new prefs are stored in the
local state - one for timer start time and the other for warning time.
According to the policy requirements, the warning time for a user
cannot decrease for good user experience.

When a new policy is received and update is required, the policy
handler calculates the new deadline using the already existing timer
start time from the local state (if it exists). The local state is
updated with the new pref values and a timer is started to expire when
deadline is reached which then blocks user session. The policy handler
also observes BuildState for any updates in Chrome version. If Chrome
is updated before deadline, the timer is stopped and update is no
longer required.

The CL does not fully implement the policy.
Subsequent CLs will handle other cases of this policy like showing
in-session notifications in case of EOL and network limitations.

Bug: 1048607
Change-Id: Iddd3966a316b4f48417c3b8aeddefd70597f6799
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2189150
Commit-Queue: Saurabh Nijhara <snijhara@google.com>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Sergey Poromov <poromov@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#769923}