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- Remove focus stealer and directly attach ax tree to contents view of
  widget delegate of focused widget.
- Returns contents view of widget delegate as focused view if no view is
  focused.

Bug: 852653
Test: Enable ChromeVox arc native support and confirm that it works.
Change-Id: Ib370a6a2efa11d04211d8dbeae92b95a7e125dfb
Reviewed-on: https://chromium-review.googlesource.com/1117720Reviewed-by: David Reveman <reveman@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Yuki Awano <yawano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571035}