Revert change that causes performance regression in Slack

Bug: 936944
Change-Id: I3eed83b7c69b804f577ba6ea9097a6be205fc898
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1506758
Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638331}

third_party/blink/renderer/modules/accessibility/ax_object_cache_impl.cc

@@ -730,7 +730,9 @@ void AXObjectCacheImpl::FocusableChanged(Element* element) {
     ChildrenChanged(element->parentNode());
   } else {
     // Refresh the focusable state on the exposed object.
-    MarkAXObjectDirty(obj, false);
+    // TODO(accessibility) find out why using MarkAXObjectDirty(obj, false)
+    // regresses Slack performance, see https://crbug.com/936944.
+    PostNotification(obj, ax::mojom::Event::kValueChanged);
   }
 }