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Implements virtual view targeting for accessibility actions for
|TreeView| nodes so that they may be activated using the accessibility
review cursor. Previously, all actions were routed to the selected node,
even if the review cursor was on a different element.

Since focus is an accessible action, this changelist also includes
improvements to how accessibility focus events get fired around virtual
views:

- If a focused virtual descendant is set when |FocusManager| gives focus
  to the owner view, the descendant (instead of the owner view's
  accessible object) now fires the accessibility focus event.

- Consequently, by setting the |TreeView|'s focused virtual descendant,
  ahead of time, TreeView::OnFocus() no longer needs to redirect focus
  to a virtual view via ViewAccessibility::OverrideFocus(). This avoids
  firing multiple accessibility focus events for a single focus change.

             inside tree views.

AX-Relnotes: In Views, assistive technologies can now select tree items
Bug: 811277
Change-Id: I232c2acf2f4fec79ea30dbe82b5f077a4f9c32b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2135205
Commit-Queue: Brian Liu Xu <brx@microsoft.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770851}