GitLab

Adds an AXLiveRegionTracker class to keep track of live regions
in an AXTree. Uses it to fix a bug where we weren't firing the
LIVE_REGION_CHANGED event on the live root when a node was removed,
only when a node was added or changed.

I'm going to follow this up with code that optionally computes
the text of a live region change, that we can use on Android,
Chrome OS, and some older versions of macOS. So AXLiveRegionTracker
is simple now, but it will be a convenient place to put that
logic.

Bug: 560599, 930763
Change-Id: Iaff11e5adbdde533b8868226ed563567edc5589e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1464325
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727589}