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Instead of firing the signal on the node which contains the selection
focus, fire it on the first common ancestor of the anchor and focus
nodes. This means that if the AT does a tree dive to get the full
selection it won't first have to climb to find the full selection
extents.

Also, along with this new way of firing events we also take into account
the anchor and focus nodes when deciding whether or not to avoid firing
duplicate caret-changed events.

Bug: 1003359
Change-Id: Ia4ac03be3e0548e9b435d6e21512c3c22459e6ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1816474Reviewed-by: Joanmarie Diggs <jdiggs@igalia.com>
Commit-Queue: Martin Robinson <mrobinson@igalia.com>
Cr-Commit-Position: refs/heads/master@{#701039}