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Make the ChromeVox hint dialog an oobe-modal-dialog in accordance with
the following change: http://crrev.com/c/2563481. This ensures that the
shelf buttons cannot be acted on while the dialog is open.

Bug: N/A
Change-Id: Ie08bfc44fdc6eb73e803c2ec10449cc461e455c2
AX-Relnotes: N/A
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577545
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834286}