Oilpan:Allow custom handling of classes that contain weak pointers that are embedded in collections.

Source/wtf/HashTraits.h

Index: Source/wtf/HashTraits.h
diff --git a/Source/wtf/HashTraits.h b/Source/wtf/HashTraits.h
index adca0f85335db622fe6dce98a7a7c3239620bb9b..09ae26480a43a19e4abde338b0522f0c8dcd8aa7 100644
--- a/Source/wtf/HashTraits.h
+++ b/Source/wtf/HashTraits.h
@@ -64,6 +64,8 @@ namespace WTF {
             static const bool value = NeedsTracing<T>::value;
         };
         static const bool isWeak = IsWeak<T>::value;
+        template<typename Visitor>
+        static bool shouldRemoveFromCollection(Visitor*, T&) { return false; }
     };
 
     // Default integer traits disallow both 0 and -1 as keys (max value instead of -1 for unsigned).
@@ -276,6 +278,12 @@ namespace WTF {
 
         static void constructDeletedValue(TraitType& slot) { KeyTraits::constructDeletedValue(slot.key); }
         static bool isDeletedValue(const TraitType& value) { return KeyTraits::isDeletedValue(value.key); }
+        template<typename Visitor>
+        static bool shouldRemoveFromCollection(Visitor* visitor, TraitType& pair)
+        {
+            return KeyTraits::shouldRemoveFromCollection(visitor, pair.key)
+                || ValueTraits::shouldRemoveFromCollection(visitor, pair.value);
+        }
     };
 
     template<typename Key, typename Value>