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Switches to using an accessibility role of alert dialog which is more appropriate for a notification toast than a window role.
Alert dialogs are widgets that may contain interactive controls that need to be announced by a screen reader as soon as they appear.
Also, an alert event should be fired regardless to whether the notification has been updated or has been initiallly set.
R=aleventhal@chromium.org, dmazzoni@chromium.org, dewittj@chromium.org, peter@chromium.org

Tested: Manually with Jaws and NVDA screen readers on Windows
Change-Id: Ida3abe94b9dc32a5c95ce6b14c6fac11670348e3
Reviewed-on: https://chromium-review.googlesource.com/968672Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545510}