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

Source/platform/heap/HeapAllocator.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef HeapAllocator_h
 #define HeapAllocator_h
 
 #include "platform/heap/Heap.h"
 #include "platform/heap/TraceTraits.h"
 #include "wtf/Assertions.h"
@@ skipping 261 matching lines @@
 public:
     static void finalize(void* pointer);
     void finalizeGarbageCollectedObject() { finalize(this); }
 };
 
 template<typename T, typename Traits>
 void HeapVectorBacking<T, Traits>::finalize(void* pointer)
 {
     static_assert(Traits::needsDestruction, "Only vector buffers with items requiring destruction should be finalized");
     // See the comment in HeapVectorBacking::trace.
-    static_assert(Traits::canInitializeWithMemset || WTF::IsPolymorphic<T>::value, "HeapVectorBacking doesn't support objects that cannot be initialized with memset or don't have a vtable");
+    static_assert(Traits::canClearUnusedSlotsWithMemset || WTF::IsPolymorphic<T>::value, "HeapVectorBacking doesn't support objects that cannot be cleared as unused with memset or don't have a vtable");
 
     ASSERT(!WTF::IsTriviallyDestructible<T>::value);
     HeapObjectHeader* header = HeapObjectHeader::fromPayload(pointer);
     header->checkHeader();
     // Use the payload size as recorded by the heap to determine how many
     // elements to finalize.
     size_t length = header->payloadSize() / sizeof(T);
     T* buffer = reinterpret_cast<T*>(pointer);
 #ifdef ANNOTATE_CONTIGUOUS_CONTAINER
     // As commented above, HeapVectorBacking calls finalizers for unused slots
@@ skipping 164 matching lines @@
 template<typename T, size_t i, typename U>
 inline void swap(HeapListHashSet<T, i, U>& a, HeapListHashSet<T, i, U>& b) { a.swap(b); }
 template<typename T, typename U, typename V>
 inline void swap(HeapLinkedHashSet<T, U, V>& a, HeapLinkedHashSet<T, U, V>& b) { a.swap(b); }
 template<typename T, typename U, typename V>
 inline void swap(HeapHashCountedSet<T, U, V>& a, HeapHashCountedSet<T, U, V>& b) { a.swap(b); }
 
 } // namespace blink
 
 #endif