Fire live region change events in UI on a slight delay

When a live region event is fired at the same time as other events, JAWS
is preferring to to skip the live region announcement. Firing the
live region event on a slight delay fixes this problem.

Bug: 1024847
Change-Id: If1eacb95d3cbbf7bbc58c54dfb1a3f64eda76ae1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1917339Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715819}

ui/views/accessibility/view_ax_platform_node_delegate.cc

@@ -99,6 +99,14 @@ void FlushQueue() {
   g_event_queue.Get().clear();
 }
 
+void PostFlushEventQueueTaskIfNecessary() {
+  if (!g_is_queueing_events) {
+    g_is_queueing_events = true;
+    base::OnceCallback<void()> cb = base::BindOnce(&FlushQueue);
+    base::ThreadTaskRunnerHandle::Get()->PostTask(FROM_HERE, std::move(cb));
+  }
+}
+
 }  // namespace
 
 struct ViewAXPlatformNodeDelegate::ChildWidgetsResult {
@@ -164,11 +172,16 @@ void ViewAXPlatformNodeDelegate::NotifyAccessibilityEvent(
       // before the next focus event. It makes sense to delay the entire next
       // synchronous batch of next events so that ordering remains the same.
       // Begin queueing subsequent events and flush queue asynchronously.
-      g_is_queueing_events = true;
-      base::OnceCallback<void()> cb = base::BindOnce(&FlushQueue);
-      base::ThreadTaskRunnerHandle::Get()->PostTask(FROM_HERE, std::move(cb));
+      PostFlushEventQueueTaskIfNecessary();
       break;
     }
+    case ax::mojom::Event::kLiveRegionChanged: {
+      // Fire after a delay so that screen readers don't wipe it out when
+      // another user-generated event fires simultaneously.
+      PostFlushEventQueueTaskIfNecessary();
+      g_event_queue.Get().emplace_back(event_type, GetUniqueId());
+      return;
+    }
     default:
       break;
   }