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Updates UpdateScreen browser tests to test UI state changes in addition
to interactions with Chrome OS system state.

Adds coverage for testing the screen state when update requires user
confirmation to proceed (when the current network connection is
detected as metered), and reboot request timeout.

To make testing easier:
*   extracts logic for setting up network portal
    state into NetworkPortalDetectorMixin (this will be reused for
    other tests).
*   refactors OobeScreen waiter to have only one Wait method, and wait
    parameters are expected to be set before a call to Wait()
    *   introduces an option to confirm that no other oobe screen is
        shown before the target screen
    *   remove unused WaitForInitialization

BUG=945133

Change-Id: I1c9899f9adad6d70a5de5d206fa1d3adf2f7e06f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1536112Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644663}