Bug 1618364 - Add OSX accessible lifecycle notifications. r=morgan

Differential Revision: https://phabricator.services.mozilla.com/D64481

--HG--
extra : moz-landing-system : lando

accessible/mac/mozAccessible.mm

@@ -1142,9 +1142,16 @@ struct RoleDescrComparator {
   if (curNative) [mChildren addObject:curNative];
 }
 
+- (BOOL)accessibilityNotifiesWhenDestroyed {
+  return YES;
+}
+
 - (void)expire {
   NS_OBJC_BEGIN_TRY_ABORT_BLOCK;
 
+  NSAccessibilityPostNotification(
+      self, NSAccessibilityUIElementDestroyedNotification);
+
   [self invalidateChildren];
 
   mGeckoAccessible = 0;