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This change refactors parts of the Accessibility Bridge to solve the
following bugs: * Previously, it was deleting nodes that were marked as
reparented or a subtree that was reparented. This assumed that nodes
marked this way would appear in the list of updates sent by the
AxTreeObserver in the final atomic update, but this assumption is not
correct. This fixes the problem where parents were pointing to children
no longer in the tree.

* Allow trees of size 1 to be sent. Previously, the loop that dealt with
  sending updates started from the second item, which would not send
  anything if the tree had only one element.

* Simplifies the order of updates: first deletions and then
  updates. Because atomic updates always finish with the list of nodes
  added / changed, we can use this property to simplify the code to
  always follow this order.

Todos: There are some todos left in the code which I opted to follow-up
in a future cl so that I can split work among people and parallelize.

Test: AccessibilityBridgeTest.*
Bug: 1134384,fuchsia:58989,fuchsia:50187,fuchsia:61155
Change-Id: I6ec09a5d6fa5d5ad37fb42a059d0d0f122d39d97
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2446358
Commit-Queue: Lucas Radaelli <lucasradaelli@google.com>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Sharon Yang <yangsharon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815346}