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Due to changes in the API, AccessibleNode will no longer be able to
modify the underlying accessibility tree. For more information on the
motivations for this, see:

https://github.com/WICG/aom/blob/gh-pages/explainer.md#what-happened-to-accessiblenode

BUG=746523
https://github.com/WICG/aom/blob/gh-pages/explainer.md

Change-Id: Ia8fd5c6994e97e317a7360739a8a9d0d6d246fa6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1573299Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Meredith Lane <meredithl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658979}