Add a pure virtual method to the GarbageCollectedMixin class

Source/platform/heap/Visitor.h

Index: Source/platform/heap/Visitor.h
diff --git a/Source/platform/heap/Visitor.h b/Source/platform/heap/Visitor.h
index 8e6218c8d15dfea8e30689374b99d1a90117ead9..cd073daf5c73a3fcf02d5e476afcda9f66d662d7 100644
--- a/Source/platform/heap/Visitor.h
+++ b/Source/platform/heap/Visitor.h
@@ -585,8 +585,8 @@ template<typename T> bool ObjectAliveTrait<T>::isHeapObjectAlive(Visitor* visito
 
 class PLATFORM_EXPORT GarbageCollectedMixin {
 public:
-    virtual void adjustAndMark(Visitor*) const { };
-    virtual bool isHeapObjectAlive(Visitor*) const { return true; };
+    virtual void adjustAndMark(Visitor*) const = 0;
+    virtual bool isHeapObjectAlive(Visitor*) const = 0;
     virtual void trace(Visitor*) { }
 };