Introduce flat_tree_children_ in preparation for reducing the number of will-be-reattached nodes

This CL is just a refactoring so that we can lazyReattach nodes in once place.

In follow-up CLs, I'll reduce the number of LazyReattached nodes, unifying this with
dynamic programming code as HTMLSlotElement::LazyReattachDistributedNodesIfNeeded does.

Bug: 776656
Change-Id: I17813a39c57cc5bbea5e8c529805123ccabd4c69
Reviewed-on: https://chromium-review.googlesource.com/984917Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Takayoshi Kochi <kochi@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546771}

third_party/WebKit/Source/core/dom/SlotAssignment.cpp

@@ -237,8 +237,6 @@ void SlotAssignment::RecalcAssignmentNg() {
   for (Node& child : NodeTraversal::ChildrenOf(owner_->host())) {
     if (!child.IsSlotable())
       continue;
-    // TODO(hayato): Avoid unconditional LazyReattach
-    child.LazyReattachIfAttached();
 
     HTMLSlotElement* slot = nullptr;
     if (!is_user_agent) {
@@ -253,11 +251,7 @@ void SlotAssignment::RecalcAssignmentNg() {
 
     if (slot)
       slot->AppendAssignedNode(child);
-  }
-
-  // TODO(hayato): Avoid unconditional LazyReattach
-  for (Member<HTMLSlotElement> slot : Slots()) {
-    for (Node& child : NodeTraversal::ChildrenOf(*slot))
+    else
       child.LazyReattachIfAttached();
   }
 
@@ -266,6 +260,9 @@ void SlotAssignment::RecalcAssignmentNg() {
         .GetSlotAssignmentEngine()
         .RemoveShadowRootNeedingRecalc(*owner_);
   }
+
+  for (auto& slot : Slots())
+    slot->RecalcFlatTreeChildren();
 }
 
 void SlotAssignment::RecalcAssignment() {

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

@@ -230,13 +230,33 @@ void HTMLSlotElement::AppendDistributedNodesFrom(const HTMLSlotElement& other) {
 
 void HTMLSlotElement::ClearAssignedNodes() {
   DCHECK(RuntimeEnabledFeatures::IncrementalShadowDOMEnabled());
-  // TODO(hayato): Avoid uncondional LazyReattach
-  for (const auto& node : assigned_nodes_)
-    node->LazyReattachIfAttached();
-
   assigned_nodes_.clear();
 }
 
+void HTMLSlotElement::RecalcFlatTreeChildren() {
+  DCHECK(RuntimeEnabledFeatures::IncrementalShadowDOMEnabled());
+  DCHECK(SupportsAssignment());
+
+  HeapVector<Member<Node>> old_flat_tree_children;
+  old_flat_tree_children.swap(flat_tree_children_);
+
+  if (assigned_nodes_.IsEmpty()) {
+    // Use children as fallback
+    for (auto& child : NodeTraversal::ChildrenOf(*this))
+      flat_tree_children_.push_back(child);
+  } else {
+    flat_tree_children_ = assigned_nodes_;
+  }
+
+  // Tentative naive version.
+  // TODO(hayato): Optimize this, as we do in
+  // |LazyReattachDistributedNodesIfNeeded|.
+  for (auto& node : old_flat_tree_children)
+    node->LazyReattachIfAttached();
+  for (auto& node : flat_tree_children_)
+    node->LazyReattachIfAttached();
+}
+
 void HTMLSlotElement::ClearDistribution() {
   assigned_nodes_.clear();
   distributed_nodes_.clear();
@@ -642,6 +662,7 @@ int HTMLSlotElement::tabIndex() const {
 
 void HTMLSlotElement::Trace(blink::Visitor* visitor) {
   visitor->Trace(assigned_nodes_);
+  visitor->Trace(flat_tree_children_);
   visitor->Trace(distributed_nodes_);
   visitor->Trace(old_distributed_nodes_);
   visitor->Trace(distributed_indices_);

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

@@ -124,6 +124,7 @@ class CORE_EXPORT HTMLSlotElement final : public HTMLElement {
 
   // For Incremental Shadow DOM
   void ClearAssignedNodes();
+  void RecalcFlatTreeChildren();
 
   static AtomicString NormalizeSlotName(const AtomicString&);
 
@@ -159,6 +160,9 @@ class CORE_EXPORT HTMLSlotElement final : public HTMLElement {
 
   HeapVector<Member<Node>> assigned_nodes_;
 
+  // For IncrementalShadowDOM
+  HeapVector<Member<Node>> flat_tree_children_;
+
   // For Non-IncrmentalShadowDOM. IncremntalShadowDOM never use these members.
   HeapVector<Member<Node>> distributed_nodes_;
   HeapVector<Member<Node>> old_distributed_nodes_;