Remove abstraction which is no longer necessary

Use AssignedNodes() directly. The abstraction here is no longer
necessary since we removed IncrementalShadowDOM flag.

Bug: 887827
Change-Id: Id5c7e4139ad89ce2ade6efd62bdfc1cbb50c0340
Reviewed-on: https://chromium-review.googlesource.com/1249467Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594976}

third_party/blink/renderer/core/html/html_slot_element.cc

@@ -314,7 +314,7 @@ void HTMLSlotElement::AttachLayoutTree(AttachContext& context) {
   if (SupportsAssignment()) {
     AttachContext children_context(context);
 
-    for (auto& node : ChildrenInFlatTreeIfAssignmentIsSupported()) {
+    for (auto& node : AssignedNodes()) {
       if (node->NeedsAttach())
         node->AttachLayoutTree(children_context);
     }
@@ -323,14 +323,6 @@ void HTMLSlotElement::AttachLayoutTree(AttachContext& context) {
   }
 }
 
-// TODO(hayato): Rename this function once we enable IncrementalShadowDOM
-// by default because this function doesn't consider fallback elements in case
-// of IncementalShadowDOM.
-const HeapVector<Member<Node>>&
-HTMLSlotElement::ChildrenInFlatTreeIfAssignmentIsSupported() {
-  return AssignedNodes();
-}
-
 void HTMLSlotElement::DetachLayoutTree(const AttachContext& context) {
   if (SupportsAssignment()) {
     const HeapVector<Member<Node>>& flat_tree_children = assigned_nodes_;
@@ -345,13 +337,11 @@ void HTMLSlotElement::RebuildDistributedChildrenLayoutTrees(
   if (!SupportsAssignment())
     return;
 
-  const HeapVector<Member<Node>>& flat_tree_children =
-      ChildrenInFlatTreeIfAssignmentIsSupported();
+  const HeapVector<Member<Node>>& assigned_nodes = AssignedNodes();
 
   // This loop traverses the nodes from right to left for the same reason as the
   // one described in ContainerNode::RebuildChildrenLayoutTrees().
-  for (auto it = flat_tree_children.rbegin(); it != flat_tree_children.rend();
-       ++it) {
+  for (auto it = assigned_nodes.rbegin(); it != assigned_nodes.rend(); ++it) {
     RebuildLayoutTreeForChild(*it, whitespace_attacher);
   }
 }

third_party/blink/renderer/core/html/html_slot_element.h

@@ -128,8 +128,6 @@ class CORE_EXPORT HTMLSlotElement final : public HTMLElement {
 
   bool HasSlotableChild() const;
 
-  const HeapVector<Member<Node>>& ChildrenInFlatTreeIfAssignmentIsSupported();
-
   void LazyReattachNodesIfNeeded(const HeapVector<Member<Node>>& nodes1,
                                  const HeapVector<Member<Node>>& nodes2);
   static void LazyReattachNodesNaive(const HeapVector<Member<Node>>& nodes1,