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Splits code that loops over accessibility events from the code that
loops over dirty nodes that need to be serialized. Keeps track of
duplicate ids and avoids serializing the same node twice.

This is a different approach than crrev.com/c/1063007
"Avoid serializing the same accessibility node twice in the
same message" which tried to put all of the nodes in
the same AXTreeUpdate. That approach could fail in rare cases when
multiple updates both tried to set node_id_to_clear.
Now we've refactored the event message so that it can
contain multiple AXTreeUpdates, so it's safe to just skip
an entire update if it's a node we've already serialized
in this event bundle.

Bug: 651614, 845778

Change-Id: I3cc859e7a63d9c8b8beeb930d96aa53898940485
Reviewed-on: https://chromium-review.googlesource.com/1097423
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568195}