cc: Add UMA histogram for the number of render surfaces

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 5bfb96ab4261a7a34b372078d5e78b583056821b..1fd1015c853dfaecdae817f175673c169aa13f27 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -1054,10 +1054,6 @@ DrawResult LayerTreeHostImpl::PrepareToDraw(FrameData* frame) {
   if (input_handler_client_)
     input_handler_client_->ReconcileElasticOverscrollAndRootScroll();
 
-  UMA_HISTOGRAM_CUSTOM_COUNTS(
-      "Compositing.NumActiveLayers",
-      base::saturated_cast<int>(active_tree_->NumLayers()), 1, 400, 20);
-
   if (const char* client_name = GetClientNameForMetrics()) {
     size_t total_picture_memory = 0;
     for (const PictureLayerImpl* layer : active_tree()->picture_layers())
@@ -1070,6 +1066,9 @@ DrawResult LayerTreeHostImpl::PrepareToDraw(FrameData* frame) {
                              client_name),
           base::saturated_cast<int>(total_picture_memory / 1024));
     }
+    UMA_HISTOGRAM_CUSTOM_COUNTS(

[Alexei Svitkine (slow), 2016/04/20 15:40:30]

Please add the same comment above this block as there is on line 1062.

[ajuma, 2016/04/20 15:52:10]

Done.

+        base::StringPrintf("Compositing.%s.NumActiveLayers", client_name),
+        base::saturated_cast<int>(active_tree_->NumLayers()), 1, 400, 20);
   }
 
   bool update_lcd_text = false;