Do not allocate SlotAssignment unless a shadowroot has a slot

third_party/WebKit/Source/core/dom/shadow/ShadowRoot.h

Index: third_party/WebKit/Source/core/dom/shadow/ShadowRoot.h
diff --git a/third_party/WebKit/Source/core/dom/shadow/ShadowRoot.h b/third_party/WebKit/Source/core/dom/shadow/ShadowRoot.h
index 90132e75afcbb06ad3f0af36413a17e7c266bb38..420e4a08172bf2773c0d06cab780e0987ba76dd9 100644
--- a/third_party/WebKit/Source/core/dom/shadow/ShadowRoot.h
+++ b/third_party/WebKit/Source/core/dom/shadow/ShadowRoot.h
@@ -123,7 +123,15 @@ class CORE_EXPORT ShadowRoot final : public DocumentFragment, public TreeScope {
   void registerScopedHTMLStyleChild();
   void unregisterScopedHTMLStyleChild();
 
-  SlotAssignment& ensureSlotAssignment();
+  SlotAssignment& slotAssignment() {
+    DCHECK(m_slotAssignment);
+    return *m_slotAssignment;
+  }
+
+  HTMLSlotElement* assignedSlotFor(const Node&);
+  void didAddSlot(HTMLSlotElement&);
+  void didChangeHostChildSlotName(const AtomicString& oldValue,
+                                  const AtomicString& newValue);
 
   void distributeV1();
 
@@ -155,6 +163,7 @@ class CORE_EXPORT ShadowRoot final : public DocumentFragment, public TreeScope {
   void childrenChanged(const ChildrenChange&) override;
 
   ShadowRootRareDataV0& ensureShadowRootRareDataV0();
+  SlotAssignment& ensureSlotAssignment();
 
   void addChildShadowRoot() { ++m_childShadowRootCount; }
   void removeChildShadowRoot() {