UMA metric for LayerUpdateTimes

cc/trees/layer_tree.h

Index: cc/trees/layer_tree.h
diff --git a/cc/trees/layer_tree.h b/cc/trees/layer_tree.h
index e58e71cb1f48f15d784d9d290a5c8c88ecefdee4..09ecc0c746be34667d5a4f4c2084a440008c9ac2 100644
--- a/cc/trees/layer_tree.h
+++ b/cc/trees/layer_tree.h
@@ -145,6 +145,8 @@ class CC_EXPORT LayerTree : public MutatorHostClient {
   void UnregisterLayer(Layer* layer);
   Layer* LayerById(int id) const;
 
+  size_t NumLayers() const;
+
   bool UpdateLayers(const LayerList& update_layer_list,
                     bool* content_is_suitable_for_gpu);
   bool in_paint_layer_contents() const { return in_paint_layer_contents_; }