Initial attempt at counting layers in the compositor thread.

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 <limits>
 #include <set>
 
 #include "base/debug/trace_event.h"
@@ skipping 1359 matching lines @@
 }
 
 void LayerTreeImpl::UnregisterPictureLayerImpl(PictureLayerImpl* layer) {
   layer_tree_host_impl_->UnregisterPictureLayerImpl(layer);
 }
 
 void LayerTreeImpl::InputScrollAnimationFinished() {
   layer_tree_host_impl_->ScrollEnd();
 }
 
+size_t LayerTreeImpl::NumLayers() {
+  return layer_id_map_.size();
+}
+
 }  // namespace cc