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

Source/platform/heap/Handle.h

Index: Source/platform/heap/Handle.h
diff --git a/Source/platform/heap/Handle.h b/Source/platform/heap/Handle.h
index 6a46cef73a1b9bbf1ee303b986977ceb9d7d81f2..2d272b857c2ca78768a32c773a0590f76426efc4 100644
--- a/Source/platform/heap/Handle.h
+++ b/Source/platform/heap/Handle.h
@@ -1040,6 +1040,12 @@ template<typename T> struct HashTraits<WebCore::WeakMember<T> > : SimpleClassHas
 
     static PeekOutType peek(const WebCore::WeakMember<T>& value) { return value; }
     static PassOutType passOut(const WebCore::WeakMember<T>& value) { return value; }
+    static bool shouldRemoveFromCollection(WebCore::Visitor* visitor, WebCore::WeakMember<T>& value) { return !visitor->isAlive(value); }
+    static void traceInCollection(WebCore::Visitor* visitor, WebCore::WeakMember<T>& weakMember, WebCore::ShouldWeakPointersBeMarkedStrongly strongify)
+    {
+        if (strongify == WebCore::WeakPointersActStrong)
+            visitor->trace(reinterpret_cast<WebCore::Member<T>&>(weakMember)); // Strongified visit.
+    }
 };
 
 template<typename T> struct PtrHash<WebCore::Member<T> > : PtrHash<T*> {