ComposedTreeWalker does't have to return a node if an insertion point has no distributed nodes.

Source/core/dom/shadow/ComposedTreeWalker.cpp

Index: Source/core/dom/shadow/ComposedTreeWalker.cpp
diff --git a/Source/core/dom/shadow/ComposedTreeWalker.cpp b/Source/core/dom/shadow/ComposedTreeWalker.cpp
index 26c2e25dc77442e87a87909f810205ad37066a72..0b9dc041bb5c92c3703b39e566f4c2207c604d51 100644
--- a/Source/core/dom/shadow/ComposedTreeWalker.cpp
+++ b/Source/core/dom/shadow/ComposedTreeWalker.cpp
@@ -31,6 +31,7 @@
 #include "core/dom/Element.h"
 #include "core/dom/shadow/ElementShadow.h"
 #include "core/dom/shadow/InsertionPoint.h"
+#include "core/html/shadow/HTMLContentElement.h"
 #include "core/html/shadow/HTMLShadowElement.h"
 
 namespace WebCore {
@@ -73,7 +74,8 @@ Node* ComposedTreeWalker::traverseNode(const Node* node, TraversalDirection dire
     const InsertionPoint* insertionPoint = toInsertionPoint(node);
     if (Node* found = traverseDistributedNodes(direction == TraversalDirectionForward ? insertionPoint->first() : insertionPoint->last(), insertionPoint, direction))
         return found;
-    return traverseLightChildren(node, direction);
+    ASSERT(isHTMLShadowElement(node) || (isHTMLContentElement(node) && !node->hasChildNodes()));
+    return 0;
 }
 
 Node* ComposedTreeWalker::traverseDistributedNodes(const Node* node, const InsertionPoint* insertionPoint, TraversalDirection direction)