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

Source/platform/heap/Handle.h

 /*
  * Copyright (C) 2014 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 1074 matching lines @@
     Member<T> m_object;
 };
 
 } // namespace blink
 
 namespace WTF {
 
 template <typename T> struct VectorTraits<blink::Member<T>> : VectorTraitsBase<blink::Member<T>> {
     static const bool needsDestruction = false;
     static const bool canInitializeWithMemset = true;
+    static const bool canClearUnusedSlotsWithMemset = true;
     static const bool canMoveWithMemcpy = true;
 };
 
 template <typename T> struct VectorTraits<blink::WeakMember<T>> : VectorTraitsBase<blink::WeakMember<T>> {
     static const bool needsDestruction = false;
     static const bool canInitializeWithMemset = true;
+    static const bool canClearUnusedSlotsWithMemset = true;
     static const bool canMoveWithMemcpy = true;
 };
 
 template <typename T> struct VectorTraits<blink::HeapVector<T, 0>> : VectorTraitsBase<blink::HeapVector<T, 0>> {
     static const bool needsDestruction = false;
     static const bool canInitializeWithMemset = true;
+    static const bool canClearUnusedSlotsWithMemset = true;
     static const bool canMoveWithMemcpy = true;
 };
 
 template <typename T> struct VectorTraits<blink::HeapDeque<T, 0>> : VectorTraitsBase<blink::HeapDeque<T, 0>> {
     static const bool needsDestruction = false;
     static const bool canInitializeWithMemset = true;
+    static const bool canClearUnusedSlotsWithMemset = true;
     static const bool canMoveWithMemcpy = true;
 };
 
 template <typename T, size_t inlineCapacity> struct VectorTraits<blink::HeapVector<T, inlineCapacity>> : VectorTraitsBase<blink::HeapVector<T, inlineCapacity>> {
     static const bool needsDestruction = VectorTraits<T>::needsDestruction;
     static const bool canInitializeWithMemset = VectorTraits<T>::canInitializeWithMemset;
+    static const bool canClearUnusedSlotsWithMemset = VectorTraits<T>::canClearUnusedSlotsWithMemset;
     static const bool canMoveWithMemcpy = VectorTraits<T>::canMoveWithMemcpy;
 };
 
 template <typename T, size_t inlineCapacity> struct VectorTraits<blink::HeapDeque<T, inlineCapacity>> : VectorTraitsBase<blink::HeapDeque<T, inlineCapacity>> {
     static const bool needsDestruction = VectorTraits<T>::needsDestruction;
     static const bool canInitializeWithMemset = VectorTraits<T>::canInitializeWithMemset;
+    static const bool canClearUnusedSlotsWithMemset = VectorTraits<T>::canClearUnusedSlotsWithMemset;
     static const bool canMoveWithMemcpy = VectorTraits<T>::canMoveWithMemcpy;
 };
 
 template<typename T> struct HashTraits<blink::Member<T>> : SimpleClassHashTraits<blink::Member<T>> {
     // FIXME: The distinction between PeekInType and PassInType is there for
     // the sake of the reference counting handles. When they are gone the two
     // types can be merged into PassInType.
     // FIXME: Implement proper const'ness for iterator types. Requires support
     // in the marking Visitor.
     using PeekInType = RawPtr<T>;
@@ skipping 146 matching lines @@
 struct ParamStorageTraits<RawPtr<T>> : public PointerParamStorageTraits<T*, blink::IsGarbageCollectedType<T>::value> {
     static_assert(sizeof(T), "T must be fully defined");
 };
 
 template<typename T>
 PassRefPtr<T> adoptRef(blink::RefCountedGarbageCollected<T>*) = delete;
 
 } // namespace WTF
 
 #endif