Fix assignedNodes({flatten:true}) to return fallback content in 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 12eec94cb5b617d39bc22179f0aaa73672cff0c4..fa0c675f8bd2a84cde0b4047402d49da0dfd8cd8 100644
--- a/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
@@ -70,19 +70,25 @@ const HeapVector<Member<Node>> HTMLSlotElement::assignedNodesForBinding(
     const AssignedNodesOptions& options) {
   UpdateDistribution();
   if (options.hasFlatten() && options.flatten())
-    return GetDistributedNodesForBinding();
+    return GetDistributedNodes();
   return assigned_nodes_;
 }
 
-const HeapVector<Member<Node>>
-HTMLSlotElement::GetDistributedNodesForBinding() {
+const HeapVector<Member<Node>> HTMLSlotElement::GetDistributedNodes() const {
   DCHECK(!NeedsDistributionRecalc());
+  DCHECK(SupportsDistribution() || distributed_nodes_.IsEmpty());
   if (SupportsDistribution())
     return distributed_nodes_;
+  // If a slot does not support distribution, its distributed_nodes_ is not
+  // pre-calculated and empty.  Instead, return fallback content for this slot.
+  // This happens only in a slot in non-shadow trees, so its assigned nodes
+  // are always empty.
+  return GetFallbackContent();
+}
 
-  // If a slot does not support distribution, its m_distributedNodes should not
-  // be used.  Instead, calculate distribution manually here. This happens only
-  // in a slot in non-shadow trees, so its assigned nodes are always empty.
+const HeapVector<Member<Node>>
+HTMLSlotElement::GetFallbackContent() const {
+  DCHECK(!SupportsDistribution() && distributed_nodes_.IsEmpty());
   HeapVector<Member<Node>> distributed_nodes;
   Node* child = NodeTraversal::FirstChild(*this);
   while (child) {
@@ -100,12 +106,6 @@ HTMLSlotElement::GetDistributedNodesForBinding() {
   return distributed_nodes;
 }
 
-const HeapVector<Member<Node>>& HTMLSlotElement::GetDistributedNodes() {
-  DCHECK(!NeedsDistributionRecalc());
-  DCHECK(SupportsDistribution() || distributed_nodes_.IsEmpty());
-  return distributed_nodes_;
-}
-
 void HTMLSlotElement::AppendAssignedNode(Node& host_child) {
   DCHECK(host_child.IsSlotable());
   assigned_nodes_.push_back(&host_child);
@@ -132,8 +132,11 @@ void HTMLSlotElement::AppendDistributedNode(Node& node) {
 
 void HTMLSlotElement::AppendDistributedNodesFrom(const HTMLSlotElement& other) {
   size_t index = distributed_nodes_.size();
-  distributed_nodes_.AppendVector(other.distributed_nodes_);
-  for (const auto& node : other.distributed_nodes_)
+  const HeapVector<Member<Node>>& other_distributed_nodes =
+      SupportsDistribution() ? other.distributed_nodes_ :
+      other.GetFallbackContent();
+  distributed_nodes_.AppendVector(other_distributed_nodes);
+  for (const auto& node : other_distributed_nodes)
     distributed_indices_.Set(node.Get(), index++);
 }