Add some tracing to most expensive accessibility functions.

content/renderer/accessibility/blink_ax_tree_source.cc

Index: content/renderer/accessibility/blink_ax_tree_source.cc
diff --git a/content/renderer/accessibility/blink_ax_tree_source.cc b/content/renderer/accessibility/blink_ax_tree_source.cc
index 4e20199df6690420804e262d3109e7c94e514955..a4596faa262012f10609280c004a3db66a1ba398 100644
--- a/content/renderer/accessibility/blink_ax_tree_source.cc
+++ b/content/renderer/accessibility/blink_ax_tree_source.cc
@@ -385,6 +385,9 @@ void BlinkAXTreeSource::SerializeNode(blink::WebAXObject src,
   dst->state = AXStateFromBlink(src);
   dst->id = src.AxID();
 
+  TRACE_EVENT1("accessibility", "BlinkAXTreeSource::SerializeNode", "role",
+               ui::ToString(dst->role));
+
   WebAXObject offset_container;
   WebFloatRect bounds_in_container;
   SkMatrix44 container_transform;