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This CL adds support for screen readers to read out when browsing data
is being cleared and when it has successfully cleared.

There seems to be an issue when the IronA11yAnnouncer's element lives
outside of a modal dialog as it is ignored by the screenreader.
Similarly, if the alert is inside of the dialog, its contents do not
get read if the dialog closes. Therefore, the "Clearing data" alert
needs to live inside the dialog as it is read while the dialog is open,
and the "Data cleared" alert needs to live outside of the dialog as it
is read after the dialog is closed.

Fixed: 1101151
Change-Id: I88c3dd98383b5d1b66a4b8492dcc8e8b817a8843
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2569942
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834495}