Initial attempt at counting layers in the compositor thread.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 8050377440ffb539d1b56570176a9c2e6dcad118..0f2801867d901b57cf162798b249839df3180566 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -1102,6 +1102,9 @@ DrawResult LayerTreeHostImpl::PrepareToDraw(FrameData* frame) {
   }
   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";