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When an AXTree of Views is added to the existing forest of accessibility trees,
and when AXPosition starts being used more on Chrome OS,
we'll need a way to determine if a particular node is at the root of
a window-like container, in order to be able to easily jump to
its beginning or its end.
Window-like containers include webpages, PDFs, browser windows, the App List,
and the whole desktop.

AX-Relnotes: n/a.

R=dmazzoni@chromium.org, aleventhal@chromium.org

Change-Id: Ic0d0d36d9aba80e50b171d82e8fc97215a4b1694
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2575259
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837458}