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Adds logic to display "persistent" error bubble (i.e. error bubble
that remains shown if user clicks, scrolls, types outside of the
bubble) that warns the user when a detachable base change / invalid
detachable base is detected. The warning warns the user that the
detachable base is different than he one they used last, and that
they should proceed with caution as the base might be malicious.

SigninScreenHandler is updated to observer ash::DetachableBaseHandler
state (currently non-mash only), and notify the UI when the error
message should be shown or hidden as the detachable base pairing
status, and the focused user pod change.

BUG=796300

Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I2813af6ac9473554509308194b3ae04331d013b9
Reviewed-on: https://chromium-review.googlesource.com/956802
Commit-Queue: Toni Barzic <tbarzic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543219}