Introduce (Heap)Vector trait covering zero'ed memory for unused slots.

Source/platform/heap/TraceTraits.h

Index: Source/platform/heap/TraceTraits.h
diff --git a/Source/platform/heap/TraceTraits.h b/Source/platform/heap/TraceTraits.h
index 6f8a3b82a79f6aa00097d1f3e180eb76eb8bb89e..a73f4c2b53cb89d429d1c4d49329da8d7d140f40 100644
--- a/Source/platform/heap/TraceTraits.h
+++ b/Source/platform/heap/TraceTraits.h
@@ -361,7 +361,7 @@ struct TraceInCollectionTrait<NoWeakHandlingInCollections, strongify, blink::Hea
         //   This is fine because the fact that the object can be initialized
         //   with memset indicates that it is safe to treat the zerod slot
         //   as a valid object.
-        static_assert(!ShouldBeTraced<Traits>::value || Traits::canInitializeWithMemset || WTF::IsPolymorphic<T>::value, "HeapVectorBacking doesn't support objects that cannot be initialized with memset.");
+        static_assert(!ShouldBeTraced<Traits>::value || Traits::canClearUnusedSlotsWithMemset || WTF::IsPolymorphic<T>::value, "HeapVectorBacking doesn't support objects that cannot be cleared as unused with memset.");
 
         // This trace method is instantiated for vectors where
         // ShouldBeTraced<Traits>::value is false, but the trace method