cc: Add UMA histogram for the number of render surfaces

cc/trees/layer_tree_host_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_host_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 1036 matching lines @@
 }
 
 DrawResult LayerTreeHostImpl::PrepareToDraw(FrameData* frame) {
   TRACE_EVENT1("cc",
                "LayerTreeHostImpl::PrepareToDraw",
                "SourceFrameNumber",
                active_tree_->source_frame_number());
   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())
       total_picture_memory += layer->GetRasterSource()->GetPictureMemoryUsage();
     if (total_picture_memory != 0) {
       // GetClientNameForMetrics only returns one non-null value over the
       // lifetime of the process, so this histogram name is runtime constant.
       UMA_HISTOGRAM_COUNTS(
           base::StringPrintf("Compositing.%s.PictureMemoryUsageKb",
                              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;
   bool ok = active_tree_->UpdateDrawProperties(update_lcd_text);
   DCHECK(ok) << "UpdateDrawProperties failed during draw";
 
   // This will cause NotifyTileStateChanged() to be called for any tiles that
   // completed, which will add damage for visible tiles to the frame for them so
   // they appear as part of the current frame being drawn.
   tile_manager_->Flush();
@@ skipping 2860 matching lines @@
   return task_runner_provider_->HasImplThread();
 }
 
 bool LayerTreeHostImpl::CommitToActiveTree() const {
   // In single threaded mode we skip the pending tree and commit directly to the
   // active tree.
   return !task_runner_provider_->HasImplThread();
 }
 
 }  // namespace cc