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So that ChromeVox and Select to Speak will continue to work with the new selection code,
anchor and focus should be swapped if focus comes before anchor in the accessibility tree.
This is achieved by introducing a new AXTreeData member, sel_is_backward,
and new selectionStart/End automation APIs.
R=dmazzoni@chromium.org, dtseng@chromium.org

Change-Id: I7e322549760c1f8028d0d7115fe0fce1c428cd1d
Bug: 639340
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1551714
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649730}