Initial attempt at counting layers in the compositor thread.

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 <algorithm>
 #include <limits>
 
 #include "base/basictypes.h"
@@ skipping 1084 matching lines @@
                "LayerTreeHostImpl::PrepareToDraw",
                "SourceFrameNumber",
                active_tree_->source_frame_number());
 
   if (need_to_update_visible_tiles_before_draw_ &&
       tile_manager_ && tile_manager_->UpdateVisibleTiles()) {
     DidInitializeVisibleTile();
   }
   need_to_update_visible_tiles_before_draw_ = true;
 
+  UMA_HISTOGRAM_CUSTOM_COUNTS(
+      "Compositing.NumActiveLayers", active_tree_->NumLayers(), 1, 400, 20);
+
   bool ok = active_tree_->UpdateDrawProperties();
   DCHECK(ok) << "UpdateDrawProperties failed during draw";
 
   frame->render_surface_layer_list = &active_tree_->RenderSurfaceLayerList();
   frame->render_passes.clear();
   frame->render_passes_by_id.clear();
   frame->will_draw_layers.clear();
   frame->contains_incomplete_tile = false;
   frame->has_no_damage = false;
 
@@ skipping 2131 matching lines @@
 }
 
 void LayerTreeHostImpl::UnregisterPictureLayerImpl(PictureLayerImpl* layer) {
   std::vector<PictureLayerImpl*>::iterator it =
       std::find(picture_layers_.begin(), picture_layers_.end(), layer);
   DCHECK(it != picture_layers_.end());
   picture_layers_.erase(it);
 }
 
 }  // namespace cc