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Write and pass a test case for an invalid AXTree. Test exposed
error handling not previously considered. This change list depends
on https://chromium-review.googlesource.com/c/chromium/src/+/1351782.

Change-Id: Ie5e65f6583a9baf6861a95e7db4969bd3ec0e979
Reviewed-on: https://chromium-review.googlesource.com/c/1357530
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613388}