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When a node with scopes_route flag is added, perform a depth first
search for a child with names_route flag.  If found, that node
should receive focus and be spoken.  Subsequent updates for a
the node should not speak or refocus.  When a scopes node is
removed, refocus a names node under the last scopes node unless
there is none. Refocused nodes after a removed node should not
be spoken, only focused.  If there is no scopes node found, focus
on the first focusable node in the tree.

This is used by flutter alert dialogs and drawers that require
the user's attention if the screen reader is enabled.

Bug: None
Test: Local display assistant and unittest
Change-Id: I69b0205d47269ec75265ca1af56400cc28faea56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436768Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Randy Rossi <rmrossi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812086}