cc: Add UMA histogram for the number of render surfaces

cc/trees/layer_tree_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 816 matching lines @@
         settings().layer_transforms_should_scale_layer_contents,
         &render_surface_layer_list_, render_surface_layer_list_id_,
         &property_trees_);
     LayerTreeHostCommon::CalculateDrawProperties(&inputs);
     if (const char* client_name = GetClientNameForMetrics()) {
       UMA_HISTOGRAM_COUNTS(
           base::StringPrintf(
               "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()));
     }
   }
 
   {
     TRACE_EVENT2("cc", "LayerTreeImpl::UpdateDrawProperties::Occlusion",
                  "IsActive", IsActiveTree(), "SourceFrameNumber",
                  source_frame_number_);
     OcclusionTracker occlusion_tracker(
         root_layer()->render_surface()->content_rect());
     occlusion_tracker.set_minimum_tracking_size(
@@ skipping 1234 matching lines @@
 }
 
 void LayerTreeImpl::ResetAllChangeTracking(PropertyTrees::ResetFlags flag) {
   layers_that_should_push_properties_.clear();
   for (auto* layer : *this)
     layer->ResetChangeTracking();
   property_trees_.ResetAllChangeTracking(flag);
 }
 
 }  // namespace cc