Clean up ui<->cc layers interface.

ui/compositor/layer.h

Index: ui/compositor/layer.h
diff --git a/ui/compositor/layer.h b/ui/compositor/layer.h
index bb8b1af6de89c7dd944136a3df2c4b080d88b62b..2fe660c5b71d1e1e052584126776a8a4447a7cc8 100644
--- a/ui/compositor/layer.h
+++ b/ui/compositor/layer.h
@@ -88,7 +88,9 @@ class COMPOSITOR_EXPORT Layer
 
   // Called by the compositor when the Layer is set as its root Layer. This can
   // only ever be called on the root layer.
-  void SetCompositor(Compositor* compositor);
+  void SetCompositor(Compositor* compositor,
+                     scoped_refptr<cc::Layer> root_layer);
+  void ResetCompositor();
 
   LayerDelegate* delegate() { return delegate_; }
   void set_delegate(LayerDelegate* delegate) { delegate_ = delegate; }
@@ -141,6 +143,8 @@ class COMPOSITOR_EXPORT Layer
   void SetTransform(const gfx::Transform& transform);
   gfx::Transform transform() const;
 
+  gfx::PointF position() const { return cc_layer_->position(); }
+
   // Return the target transform if animator is running, or the current
   // transform otherwise.
   gfx::Transform GetTargetTransform() const;
@@ -350,7 +354,7 @@ class COMPOSITOR_EXPORT Layer
       ContentLayerClient::PaintingControlSetting painting_control) override;
   bool FillsBoundsCompletely() const override;
 
-  cc::Layer* cc_layer() { return cc_layer_; }
+  cc::Layer* cc_layer_for_testing() { return cc_layer_; }
 
   // TextureLayerClient
   bool PrepareTextureMailbox(