Setup LayerTree class, refactor 2 functions from LayerTreeHost to it.

cc/layers/layer.h

Index: cc/layers/layer.h
diff --git a/cc/layers/layer.h b/cc/layers/layer.h
index 711ae7152848cdf6c9951ece43f9f6c97303cba1..d46a7045a6a10ac13a70ff2b9bf9b284e156b84b 100644
--- a/cc/layers/layer.h
+++ b/cc/layers/layer.h
@@ -27,6 +27,7 @@
 #include "cc/layers/layer_position_constraint.h"
 #include "cc/layers/paint_properties.h"
 #include "cc/output/filter_operations.h"
+#include "cc/trees/layer_tree.h"
 #include "cc/trees/mutator_host_client.h"
 #include "cc/trees/property_tree.h"
 #include "third_party/skia/include/core/SkColor.h"
@@ -393,6 +394,7 @@ class CC_EXPORT Layer : public base::RefCounted<Layer> {
 
   LayerTreeHost* layer_tree_host() { return layer_tree_host_; }
   const LayerTreeHost* layer_tree_host() const { return layer_tree_host_; }
+  LayerTree* GetLayerTree() const;

[vmpstr, 2016/07/27 18:05:30]

nit: Can you add a TODO to create an interface and this function will be a part
of it? I've talked to Khushal and I think that's the intent. 

[xingliu, 2016/07/28 16:10:24]

Done.

 
   virtual ScrollbarLayerInterface* ToScrollbarLayer();