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

Source/platform/heap/TraceTraits.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 TraceTraits_h
 #define TraceTraits_h
 
 #include "platform/heap/GCInfo.h"
 #include "platform/heap/Heap.h"
 #include "platform/heap/InlinedGlobalMarkingVisitor.h"
@@ skipping 343 matching lines @@
         //   traces only slots that are not zeroed out. This is because if
         //   the object has a vtable, the zeroed slot means that it is
         //   an unused slot (Remember that the unused slots are guaranteed
         //   to be zeroed out by VectorUnusedSlotClearer).
         //
         // - An object that can be initialized with memset. In this case,
         //   HeapVectorBacking traces all slots including unused slots.
         //   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
         // should not be called. Thus we cannot static-assert
         // ShouldBeTraced<Traits>::value but should runtime-assert it.
         ASSERT(ShouldBeTraced<Traits>::value);
 
         T* array = reinterpret_cast<T*>(self);
         blink::HeapObjectHeader* header = blink::HeapObjectHeader::fromPayload(self);
         header->checkHeader();
@@ skipping 166 matching lines @@
         // since iterating over the hash table backing will find the whole
         // chain.
         visitor->markNoTracing(node);
         return false;
     }
 };
 
 } // namespace WTF
 
 #endif