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Previously mouse-triggered focusing on an element (e.g. clicking) uses
repeated calls of ParentOrShadowElement() instead of using flat-tree
traversal methods, causing it to go up its shadow-including ancestors
instead of flat-tree. This behavior is not specified, but other
browser vendors had implemented flat-tree traversal usage for focus
traversal.

See: https://github.com/w3c/webcomponents/issues/773

Bug: 894931
Change-Id: I5666c9e973480648e8e9a1774e5d6abe026aac5c
Reviewed-on: https://chromium-review.googlesource.com/c/1455839Reviewed-by: Hayato Ito <hayato@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629515}