cc: Add UMA histogram for the number of render surfaces

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index e41da4b42d2ff4c642daddb280da95b26ebfb46d..98171259c65ddca80d1e97a01a040ba0e85b4852 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -834,6 +834,9 @@ bool LayerTreeImpl::UpdateDrawProperties(bool update_lcd_text) {
               "Compositing.%s.LayerTreeImpl.CalculateDrawPropertiesUs",
               client_name),
           timer.Elapsed().InMicroseconds());
+      UMA_HISTOGRAM_COUNTS_100(
+          base::StringPrintf("Compositing.%s.NumRenderSurfaces", client_name),
+          base::saturated_cast<int>(render_surface_layer_list_.size()));
     }
   }