Add a pure virtual method to the GarbageCollectedMixin class

Source/platform/heap/Visitor.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 567 matching lines @@
 //   USING_GARBAGE_COLLECTED_MIXIN(A)
 // };
 //
 // With the helper, as long as we are using Member<B>, TypeTrait<B> will
 // dispatch adjustAndMark dynamically to find collect addr of the object header.
 // Note that this is only enabled for Member<B>. For Member<A> which we can
 // compute the object header addr statically, this dynamic dispatch is not used.
 
 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*) { }
 };
 
 #define USING_GARBAGE_COLLECTED_MIXIN(TYPE) \
 public: \
     virtual void adjustAndMark(blink::Visitor* visitor) const override    \
     { \
         typedef WTF::IsSubclassOfTemplate<typename WTF::RemoveConst<TYPE>::Type, blink::GarbageCollected> IsSubclassOfGarbageCollected; \
         COMPILE_ASSERT(IsSubclassOfGarbageCollected::value, OnlyGarbageCollectedObjectsCanHaveGarbageCollectedMixins); \
         visitor->mark(static_cast<const TYPE*>(this), &blink::TraceTrait<TYPE>::trace); \
@@ skipping 55 matching lines @@
 struct GCInfoTrait {
     static const GCInfo* get()
     {
         return GCInfoAtBase<typename GetGarbageCollectedBase<T>::type>::get();
     }
 };
 
 }
 
 #endif