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

Source/core/layout/HitTestResult.h

Index: Source/core/layout/HitTestResult.h
diff --git a/Source/core/layout/HitTestResult.h b/Source/core/layout/HitTestResult.h
index 9deba8970168a17b170bae7776365e48c37188fc..b5bf8ecfa1f17aa1dd414bf6f003dc7255d0795c 100644
--- a/Source/core/layout/HitTestResult.h
+++ b/Source/core/layout/HitTestResult.h
@@ -173,16 +173,6 @@ private:
 
 } // namespace blink
 
-#if ENABLE(OILPAN)
-// TODO(sof): the trait override/specialization is needed by HitTestCache's
-// HeapVector<> to handle unused slots. It is not correct for HitTestResult
-// in the wider sense of what canInitializeWithMemset provides, so fix this
-// by introducing a trait that encompasses "unused slot" handling and use it
-// here instead.
-//
-// Until that time, make this trait specialization conditional on OILPAN to
-// limit exposure.
-WTF_ALLOW_INIT_WITH_MEM_FUNCTIONS(blink::HitTestResult);
-#endif
+WTF_ALLOW_CLEAR_UNUSED_SLOTS_WITH_MEM_FUNCTIONS(blink::HitTestResult);
 
 #endif // HitTestResult_h