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This uses a method suggested by aleventhal@ in that it provides a hidden
view on each widget that can be used to send accessibility notifications
via the existing (at least on Mac) ViewAccessibility::AnnounceText()
method.

This CL expands AnnounceText() to be available on all platforms, with
the non-Mac implementation using an invisible view parented to the
widget's RootView.

Currently uses alerts since they are compatible with all screen readers.
Followup work will be to change to a live region update, which is a less
obtrusive alert (see crbug.com/1024898).

Bug: 1024026
Change-Id: I9dbe0d7e38fb69d3ada8b8e1574aee211bfe6291
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1913281Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Robert Liao <robliao@chromium.org>
Auto-Submit: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/master@{#717237}