Do not allocate SlotAssignment unless a shadowroot has a slot

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

 /*
  * Copyright (C) 2015 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 186 matching lines @@
       node->lazyReattachIfAttached();
   }
   HTMLElement::detachLayoutTree(context);
 }
 
 void HTMLSlotElement::attributeChanged(
     const AttributeModificationParams& params) {
   if (params.name == nameAttr) {
     if (ShadowRoot* root = containingShadowRoot()) {
       if (root->isV1() && params.oldValue != params.newValue) {
-        root->ensureSlotAssignment().slotRenamed(
-            normalizeSlotName(params.oldValue), *this);
+        root->slotAssignment().slotRenamed(normalizeSlotName(params.oldValue),
+                                           *this);
       }
     }
   }
   HTMLElement::attributeChanged(params);
 }
 
 static bool wasInShadowTreeBeforeInserted(HTMLSlotElement& slot,
                                           ContainerNode& insertionPoint) {
   ShadowRoot* root1 = slot.containingShadowRoot();
   ShadowRoot* root2 = insertionPoint.containingShadowRoot();
   if (root1 && root2 && root1 == root2)
     return false;
   return root1;
 }
 
 Node::InsertionNotificationRequest HTMLSlotElement::insertedInto(
     ContainerNode* insertionPoint) {
   HTMLElement::insertedInto(insertionPoint);
   ShadowRoot* root = containingShadowRoot();
   if (root) {
     DCHECK(root->owner());
     root->owner()->setNeedsDistributionRecalc();
     // Relevant DOM Standard: https://dom.spec.whatwg.org/#concept-node-insert
     // - 6.4:  Run assign slotables for a tree with node's tree and a set
     // containing each inclusive descendant of node that is a slot.
     if (root->isV1() && !wasInShadowTreeBeforeInserted(*this, *insertionPoint))
-      root->ensureSlotAssignment().slotAdded(*this);
+      root->didAddSlot(*this);
   }
 
   // We could have been distributed into in a detached subtree, make sure to
   // clear the distribution when inserted again to avoid cycles.
   clearDistribution();
 
   return InsertionDone;
 }
 
 static ShadowRoot* containingShadowRootBeforeRemoved(
@@ skipping 19 matching lines @@
     if (ElementShadow* rootOwner = root->owner())
       rootOwner->setNeedsDistributionRecalc();
   }
 
   // Since this insertion point is no longer visible from the shadow subtree, it
   // need to clean itself up.
   clearDistribution();
 
   if (root && root->isV1() && root == insertionPoint->treeScope().rootNode()) {
     // This slot was in a shadow tree and got disconnected from the shadow root.
-    root->ensureSlotAssignment().slotRemoved(*this);
+    root->slotAssignment().slotRemoved(*this);
   }
 
   HTMLElement::removedFrom(insertionPoint);
 }
 
 void HTMLSlotElement::willRecalcStyle(StyleRecalcChange change) {
   if (change < IndependentInherit && getStyleChangeType() < SubtreeStyleChange)
     return;
 
   for (auto& node : m_distributedNodes)
@@ skipping 47 matching lines @@
     return;
   Microtask::enqueueMicrotask(WTF::bind(
       &HTMLSlotElement::dispatchSlotChangeEvent, wrapPersistent(this)));
   m_slotchangeEventEnqueued = true;
 }
 
 bool HTMLSlotElement::hasAssignedNodesSlow() const {
   ShadowRoot* root = containingShadowRoot();
   DCHECK(root);
   DCHECK(root->isV1());
-  SlotAssignment& assignment = root->ensureSlotAssignment();
+  SlotAssignment& assignment = root->slotAssignment();
   if (assignment.findSlotByName(name()) != this)
     return false;
   return assignment.findHostChildBySlotName(name());
 }
 
 bool HTMLSlotElement::findHostChildWithSameSlotName() const {
   ShadowRoot* root = containingShadowRoot();
   DCHECK(root);
   DCHECK(root->isV1());
-  SlotAssignment& assignment = root->ensureSlotAssignment();
+  SlotAssignment& assignment = root->slotAssignment();
   return assignment.findHostChildBySlotName(name());
 }
 
 int HTMLSlotElement::tabIndex() const {
   return Element::tabIndex();
 }
 
 DEFINE_TRACE(HTMLSlotElement) {
   visitor->trace(m_assignedNodes);
   visitor->trace(m_distributedNodes);
   visitor->trace(m_oldDistributedNodes);
   visitor->trace(m_distributedIndices);
   HTMLElement::trace(visitor);
 }
 
 }  // namespace blink