Initial attempt at counting layers in the compositor thread.

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 908e993d3ba845fcc3673c3538464f88f73a038a..bf0a99630d1a982253bd76b98c0b044c2f876973 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -561,6 +561,10 @@ void LayerTreeImpl::UnregisterLayer(LayerImpl* layer) {
   layer_id_map_.erase(layer->id());
 }
 
+size_t LayerTreeImpl::NumLayers() {
+  return layer_id_map_.size();
+}
+
 void LayerTreeImpl::PushPersistedState(LayerTreeImpl* pending_tree) {
   pending_tree->SetCurrentlyScrollingLayer(
       LayerTreeHostCommon::FindLayerInSubtree(pending_tree->root_layer(),