Port SiblingTraversalStrategies to ElementTraversal API

Source/core/dom/ElementTraversal.h

Index: Source/core/dom/ElementTraversal.h
diff --git a/Source/core/dom/ElementTraversal.h b/Source/core/dom/ElementTraversal.h
index 4235e80d45b0ab0f15809a3a2ba5a98f57dc1c13..1adba4498ceb50733935bdafff16afc0665ad229 100644
--- a/Source/core/dom/ElementTraversal.h
+++ b/Source/core/dom/ElementTraversal.h
@@ -56,7 +56,8 @@ Element* nextIncludingPseudoSkippingChildren(const Node&, const Node* stayWithin
 // Utility function to traverse only the element and pseudo-element siblings of a node.
 Element* pseudoAwarePreviousSibling(const Node&);
 
-// Next sibling.
+// Previous / Next sibling.
+Element* previousSibling(const Node&);
 Element* nextSibling(const Node&);
 
 template <class NodeType>
@@ -147,6 +148,14 @@ inline Element* pseudoAwarePreviousSibling(const Node& current)
     return toElement(node);
 }
 
+inline Element* previousSibling(const Node& current)
+{
+    Node* node = current.previousSibling();
+    while (node && !node->isElementNode())
+        node = node->previousSibling();
+    return toElement(node);
+}
+
 inline Element* nextSibling(const Node& current)
 {
     Node* node = current.nextSibling();