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This UI is shown on the settings page when a phone tells the Chromebook
that it does not have GmsCore notifications enabled.

This CL:
  (1) Updates HostScannerOperation to accept an additional enum value
      indicating that GmsCore notifications are disabled.
  (2) Updates the settings page to show an extra step which is needed in
      some cases to enable notifications.

Bug: 793936, 672263
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I65f34db384956189e6ea3c157a57f48bc4701fe4
Reviewed-on: https://chromium-review.googlesource.com/894729Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533167}