Reland of Fix FlatTreeTraversal for a slot in a document tree

third_party/WebKit/Source/core/html/HTMLSlotElement.cpp

Index: third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
diff --git a/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp b/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
index 2d4113e136e7c9ab5a5cdfb8ad5d75d4a3e4d083..1b8b7bfef1748291b272b08414b4104872c304d9 100644
--- a/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
@@ -72,11 +72,8 @@
   return m_assignedNodes;
 }
 
-const HeapVector<Member<Node>>& HTMLSlotElement::getDistributedNodes() {
-  DCHECK(!needsDistributionRecalc());
-  if (isInShadowTree())
-    return m_distributedNodes;
-
+void HTMLSlotElement::updateDistributedNodesManually() {
+  DCHECK(!supportsDistribution());
   // A slot is unlikely to be used outside of a shadow tree.
   // We do not need to optimize this case in most cases.
   // TODO(hayato): If this path causes a performance issue, we should move
@@ -96,6 +93,12 @@
       child = NodeTraversal::nextSkippingChildren(*child, this);
     }
   }
+}
+
+const HeapVector<Member<Node>>& HTMLSlotElement::getDistributedNodes() {
+  DCHECK(!needsDistributionRecalc());
+  if (!supportsDistribution())
+    updateDistributedNodesManually();
   return m_distributedNodes;
 }