cc: Make LayerTreeHost a protected Layer member.

cc/layers/layer.h

Index: cc/layers/layer.h
diff --git a/cc/layers/layer.h b/cc/layers/layer.h
index 30e9ed447c6a66e7e876dec9ca868143159d0feb..762a56ce96f27fbb9f96bf8afec3103f9169b719 100644
--- a/cc/layers/layer.h
+++ b/cc/layers/layer.h
@@ -391,8 +391,7 @@ class CC_EXPORT Layer : public base::RefCounted<Layer> {
   // of |proto| is only read if |needs_push_properties_| is set.
   void FromLayerPropertiesProto(const proto::LayerProperties& proto);
 
-  LayerTreeHost* layer_tree_host() { return layer_tree_host_; }
-  const LayerTreeHost* layer_tree_host() const { return layer_tree_host_; }
+  LayerTreeHost* GetLayerTreeHostForTesting() const { return layer_tree_host_; }
   LayerTree* GetLayerTree() const;
 
   virtual ScrollbarLayerInterface* ToScrollbarLayer();
@@ -501,6 +500,9 @@ class CC_EXPORT Layer : public base::RefCounted<Layer> {
   virtual ~Layer();
   Layer();
 
+  LayerTreeHost* layer_tree_host() { return layer_tree_host_; }
+  const LayerTreeHost* layer_tree_host() const { return layer_tree_host_; }
+
   // These SetNeeds functions are in order of severity of update:
   //
   // Called when this layer has been modified in some way, but isn't sure