Have NodeTraversal::previous*() take a reference

Source/core/dom/NodeTraversal.h

Index: Source/core/dom/NodeTraversal.h
diff --git a/Source/core/dom/NodeTraversal.h b/Source/core/dom/NodeTraversal.h
index d428e8f80f564d1a4c4faf619e142618b490ef3e..52a6c6a2a22abbbcc25f666be57ee1b1fd2f6a30 100644
--- a/Source/core/dom/NodeTraversal.h
+++ b/Source/core/dom/NodeTraversal.h
@@ -47,20 +47,20 @@ Node* nextSkippingChildren(const ContainerNode*);
 Node* nextSkippingChildren(const ContainerNode*, const Node* stayWithin);
 
 // Does a reverse pre-order traversal to find the node that comes before the current one in document order
-Node* previous(const Node*, const Node* stayWithin = 0);
+Node* previous(const Node&, const Node* stayWithin = 0);
 
 // Like previous, but skips children and starts with the next sibling.
-Node* previousSkippingChildren(const Node*, const Node* stayWithin = 0);
+Node* previousSkippingChildren(const Node&, const Node* stayWithin = 0);
 
 // Like next, but visits parents after their children.
 Node* nextPostOrder(const Node*, const Node* stayWithin = 0);
 
 // Like previous/previousSkippingChildren, but visits parents before their children.
-Node* previousPostOrder(const Node*, const Node* stayWithin = 0);
-Node* previousSkippingChildrenPostOrder(const Node*, const Node* stayWithin = 0);
+Node* previousPostOrder(const Node&, const Node* stayWithin = 0);
+Node* previousSkippingChildrenPostOrder(const Node&, const Node* stayWithin = 0);
 
 // Pre-order traversal including the pseudo-elements.
-Node* previousIncludingPseudo(const Node*, const Node* stayWithin = 0);
+Node* previousIncludingPseudo(const Node&, const Node* stayWithin = 0);
 Node* nextIncludingPseudo(const Node*, const Node* stayWithin = 0);
 Node* nextIncludingPseudoSkippingChildren(const Node*, const Node* stayWithin = 0);