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This bug could be reproduced just by visiting https://www.lasestrellas.tv/
with accessibility enabled on any platform. The issue was that reparenting
caused AXTreeSerializer to call Reset, which deleted the whole client
tree and forced re-serializing everything again. However, because some nodes
in the tree were not attached to the root at the time Reset was called,
|client_id_map_| still had some entries pointing to nodes that were no
longer valid.

The solution is for Reset() to explicitly clear out |client_id_map_|.

Bug: 704045
Change-Id: I0f1d7b24b547953970965fd49ec1a15fbade281e
Reviewed-on: https://chromium-review.googlesource.com/1006073
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550122}