The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5df8c709347dafd0c7f120a82f702fe0a422a5a5

commit 5df8c709347dafd0c7f120a82f702fe0a422a5a5
Author: David Tseng <dtseng@chromium.org>
Date: Thu Jan 17 23:47:46 2019

Improvements to active descendant handling in ChromeVox

- remove generic braille children changed event output (in favor of unifying into aria attribute changed event); this cleans up some potential observed duplicate output
- largely defer to onEventIfInRange in onActiveDescendantChanged; this inherits repeated attribute output checking/prevention.
A handler is still needed because of the unique queueing requirements for active descendant output.
- in on aria attribute changed, re-target the output to active descendant if it exists. This gives proper repeated output prevention while also keeping ChromeVox range on the containing activeDescendantFor
- this change also now makes ChromeVox remain on the top level control (and not sync range to the active descendant). This appears to be the right choice because ChromeVox cannot reliably navigate among active descendants. For example, a page author might only generate one active descendant at a time.

Bug:  921956 
Test: manual. Relies upon parent fix for event generation on re-parented nodes.
1. tested on files app (file listbox).
2. on drive.google.com, 'c', up/down arrow.

Change-Id: I2c69faef6437add90a5c36d7ac13dc552f46c0e0
Reviewed-on: https://chromium-review.googlesource.com/c/1418114
Commit-Queue: David Tseng <dtseng@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623912}
[modify] https://crrev.com/5df8c709347dafd0c7f120a82f702fe0a422a5a5/chrome/browser/resources/chromeos/chromevox/cvox2/background/desktop_automation_handler.js