UMA metric for LayerUpdateTimes

cc/trees/layer_tree.cc

Index: cc/trees/layer_tree.cc
diff --git a/cc/trees/layer_tree.cc b/cc/trees/layer_tree.cc
index bf07576cc69500ff0e9f14a4e76bfb444e1a2384..af75104fe0b93054ac9e082ee66b7ff11fde4dc2 100644
--- a/cc/trees/layer_tree.cc
+++ b/cc/trees/layer_tree.cc
@@ -241,6 +241,10 @@ Layer* LayerTree::LayerById(int id) const {
   return iter != layer_id_map_.end() ? iter->second : nullptr;
 }
 
+size_t LayerTree::NumLayers() const {
+  return layer_id_map_.size();
+}
+
 bool LayerTree::UpdateLayers(const LayerList& update_layer_list,
                              bool* content_is_suitable_for_gpu) {
   base::AutoReset<bool> painting(&in_paint_layer_contents_, true);