Build property trees for the HUD layer when using layer lists

cc/layers/layer.h

Index: cc/layers/layer.h
diff --git a/cc/layers/layer.h b/cc/layers/layer.h
index 16ff62e7b21dba0360eb9607a4c27e53c3e44dc8..564470d503d90417e3b23e2c404ff89554c84931 100644
--- a/cc/layers/layer.h
+++ b/cc/layers/layer.h
@@ -46,6 +46,7 @@ class ConvertableToTraceFormat;
 namespace cc {
 
 class CopyOutputRequest;
+class HeadsUpDisplayLayer;
 class LayerClient;
 class LayerImpl;
 class LayerTreeHost;
@@ -437,6 +438,8 @@ class CC_EXPORT Layer : public base::RefCounted<Layer> {
 
   LayerTreeHost* layer_tree_host() const { return layer_tree_host_; }
 
+  void copyPropertyTreeStateTo(HeadsUpDisplayLayer* hud_layer) const;

[ajuma, 2017/03/23 17:41:27]

not-blink-nit: Upper-case c :)

At a higher level, since we only need this for the specific case of copying root
layer state to the hud layer, I wonder if this would be better off as a helper
that did just that in LayerTreeHost rather than here.

Alternatively, if this seems like something we might want more generally, then
this should take a Layer* instead of a HeadsUpDisplayLayer*.

+
  protected:
   friend class LayerImpl;
   friend class TreeSynchronizer;