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 - Refactor and simplify next/previous unignored sibling methods on AXNode.
 - Add extensive documentation, both interface and implementation.
 - Add more extensive and finer-grained unit tests.

R=aboxhall,dmazzoni,aleventhal,janewman@microsoft.com,ethavar@microsoft.com

Change-Id: Idd85a35de6a0cfa314dfba7255c22bc58005f1e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2102269
Commit-Queue: Chris Hall <chrishall@chromium.org>
Reviewed-by: Jacques Newman <janewman@microsoft.com>
Reviewed-by: Ethan Jimenez <ethavar@microsoft.com>
Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757729}