Add TODO for missing IA2_EVENT_TEXT_REMOVED in live regions

When a generated live region event occurs, we currently fire
IA2_EVENT_TEXT_INSERTED. However, as recently discovered via
https://chromium-review.googlesource.com/c/chromium/src/+/2015429
we are not firing IA2_EVENT_TEXT_REMOVED in the generated event
case.

This currently adds a todo rather than attempting to correct the logic
for which events fire, as it is not known to cause any actual screen
reader issues.

Bug: None
Change-Id: I4d74acdf47c5909d3efe0dbf2acb9d20ffb98528
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2019410Reviewed-by: Ian Prest <iapres@microsoft.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735083}

content/browser/accessibility/browser_accessibility_manager_win.cc

@@ -282,6 +282,12 @@ void BrowserAccessibilityManagerWin::FireGeneratedEvent(
       // This will force ATs that synchronously call get_newText (e.g., NVDA) to
       // read the entire live region hypertext.
       ToBrowserAccessibilityWin(node)->GetCOM()->ForceNewHypertext();
+      // TODO(accessibility) Technically this should only be fired if the new
+      // text is non-empty. Also, IA2_EVENT_TEXT_REMOVED should be fired if
+      // there was non-empty old text. However, this does not known to affect
+      // any current screen reader behavior either way. It could affect
+      // the aria-relevant="removals" case, but that in general is poorly
+      // supported markup across browser-AT combinations, and not recommended.
       FireWinAccessibilityEvent(IA2_EVENT_TEXT_INSERTED, node);
 
       // This event is redundant with the IA2_EVENT_TEXT_INSERTED events;