No ax children changed events for caret blinks

Avoid unnecessary processor usage for blinking caret, as accessibility
children changed events are not needed. The caret is an ignored object,
meaning that it is not even exposed as a child.

These extra events are especially prevalent when the JAWS screen reader
is running, because it increases the caret blink rate in order to track
it better in legacy apps.

Bug: None
Change-Id: I83b20883dca9f73c5f529f23e5b99195bb0982a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2225621
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774608}

ui/views/view.cc

@@ -511,8 +511,10 @@ void View::SetVisible(bool visible) {
     // Notify the parent.
     if (parent_) {
       parent_->ChildVisibilityChanged(this);
-      parent_->NotifyAccessibilityEvent(ax::mojom::Event::kChildrenChanged,
-                                        true);
+      if (!view_accessibility_ || !view_accessibility_->IsIgnored()) {
+        parent_->NotifyAccessibilityEvent(ax::mojom::Event::kChildrenChanged,
+                                          true);
+      }
     }
 
     // This notifies all sub-views recursively.