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In CL:967465 we changed to exposing alertdialog from ROLE_SYSTEM_DIALOG
to ROLE_SYSTEM_ALERT, so that unfocused UI bubbles that appear would be
spoken by JAWS. However, this caused some permissions dialogs to be
spoken twice by JAWS, and caused NVDA not to read ARIA alertdialogs.

At a later point, we added xml-roles:alertdialog to these objects. This
seems to allow us to change the MSAA role back to ROLE_SYSTEM_DIALOG
while retaining the behavior we want for UI dialog bubbles, and also
fixing the regressions.

ROLE_SYSTEM_DIALOG + xml-roles:alertdialog:
- Works with JAWS dialog commands such as Insert+T to read title
  UI dialog bubbles that don't get focus.
- Fixes double speaking of permissions dialogs in JAWS
- Works in JAWS 2018, 2019, 2020
- Fixes non-reading of ARIA alertdialog in NVDA
- Complies with CORE-AAM spec and consistent with other browsers

R=nektar@chromium.org

Bug: 1071838,1007103
Change-Id: I116bb936dfd94aabedb269ae28d859956cec43c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2154391
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#761150}