Ensure Layers have valid sequence numbers for PropertyTrees

cc/layers/layer.h

Index: cc/layers/layer.h
diff --git a/cc/layers/layer.h b/cc/layers/layer.h
index ef5da3359dcdbb11e1b719f4fa0b30ceeb33d280..ced6fe59412b97abd49e8a2df06db3773bebae7d 100644
--- a/cc/layers/layer.h
+++ b/cc/layers/layer.h
@@ -470,6 +470,7 @@ class CC_EXPORT Layer : public base::RefCounted<Layer>,
   void set_property_tree_sequence_number(int sequence_number) {
     property_tree_sequence_number_ = sequence_number;
   }
+  int property_tree_sequence_number() { return property_tree_sequence_number_; }
 
   void SetTransformTreeIndex(int index);
   int transform_tree_index() const;