Avoid incomplete live region text change announcements

When a live region change occurs we sometimes fire text change events
for only part of the text. For example, if "car" changes to "cat", we
fire an event that makes it look like only the "t" changed.

This changes the heuristic so that when a live region occurs that is
not in an editor, it will prefer to indicate that entire text nodes
have changed.

Bug: 778478
Change-Id: If87f1ac8c9c0b237bd422e8b7217d8d0347a13ab
Reviewed-on: https://chromium-review.googlesource.com/1134201
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574545}
5 files changed