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When a node is reparented we currently destroy it rather than actually
reusing it, causing it to look like a new node. New nodes do not fire
change events for name, state, value or live regions.

Impact:
- We need to reuse nodes if we want them to be able to fire change
events when they are reparented at the same time.
- Accessibility nodes can be unexpectedly reparented by the layout
engine, as shown in the included test.

Bug: 469254
Change-Id: I7aec0d7852c712724b97be4a723df3c928720d62
Reviewed-on: https://chromium-review.googlesource.com/1152477
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578750}