Make classes and structures in core/dom fast-allocated.

Source/core/dom/MutationObserverRegistration.h

Index: Source/core/dom/MutationObserverRegistration.h
diff --git a/Source/core/dom/MutationObserverRegistration.h b/Source/core/dom/MutationObserverRegistration.h
index 873dd6b4872302f29a0f4ad5190a419a204e8e8d..8ff40a90310054cd052a494ef1471fac41d0e2c4 100644
--- a/Source/core/dom/MutationObserverRegistration.h
+++ b/Source/core/dom/MutationObserverRegistration.h
@@ -42,6 +42,7 @@ namespace blink {
 class QualifiedName;
 
 class MutationObserverRegistration final : public NoBaseWillBeGarbageCollectedFinalized<MutationObserverRegistration> {
+    WTF_MAKE_FAST_ALLOCATED_WILL_BE_REMOVED(MutationObserverRegistration);
 public:
     static PassOwnPtrWillBeRawPtr<MutationObserverRegistration> create(MutationObserver&, Node*, MutationObserverOptions, const HashSet<AtomicString>& attributeFilter);
     ~MutationObserverRegistration();