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This change adds support for navigation by list in ChromeVox. This
behaves in a similar manner to other navigation types such as
navigation by heading, link, button, etc.

This change also fixes bug 1012907. In this bug, ChromeVox would jump
to the second to last element, instead of the last element, when
jumping to the previous element from the top of the page.

Bug: 887630
Bug: 1012907
Change-Id: I35194c9c125e6b237db1258f8cb1a4cfdc1aa769
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1830364
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710097}