Introduce infrastructure for tracing ScriptWrappables.

third_party/WebKit/Source/bindings/core/v8/DOMWrapperMap.h

 /*
  * Copyright (C) 2012 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 75 matching lines @@
     {
         m_map.Clear();
     }
 
     void removeAndDispose(KeyType* key)
     {
         ASSERT(containsKey(key));
         m_map.Remove(key);
     }
 
+    void markWrapper(KeyType* object)
+    {
+        m_map.RegisterExternallyReferencedObject(object);

[haraken, 2016/04/18 04:35:41]

I'm okay with this, but it would be nicer if all call sites of
RegisterExternallyReferencedObject are unified into
ScriptWrappableHeapTracer::markWrapper.

If you cannot do the unification, you can keep this, but then it would make more
sense to remove ScriptWrappableHeapTracer::markWrapper. Alternately,
ScriptWrappable::markWrapper can directly call
RegisterExternallyReferencedObject. (i.e., ScriptWrappable takes care of the
main world, and DOMWrapperMap takes care of isolated worlds.)

[Marcel Hlopko, 2016/04/18 11:45:34]

I don't think we can unify this elegantly, the map lives in the V8, it's weird
to make it give us information just so we can pass it back.

I propose following: ScriptWrappableHeapTracer is THE place to use to register
external references for the v8, like a bottleneck. It has 2 markWrapper-ish
methods: one taking ScriptWrappables (called markWrappersInAllWorlds)- and
marking wrappers in all worlds, second one taking v8::persistent (called
markWrapper) - and marking only the given wrapper. Heap tracer for
markWrappersInAllWorlds, behind the scenes, delegates to
ScriptWrappable.markWrapper for main world and to DOMDataStore and that to
DOMWrapperMap for other worlds. I agree it's mess, I'm still waiting for Muse to
come and enlighten me...

At least I added the comments to reveal our intentions more.

+    }
+
 private:
     class PersistentValueMapTraits {
         STATIC_ONLY(PersistentValueMapTraits);
     public:
         // Map traits:
         typedef HashMap<KeyType*, v8::PersistentContainerValue> Impl;
         typedef typename Impl::iterator Iterator;
         static size_t Size(const Impl* impl) { return impl->size(); }
         static bool Empty(Impl* impl) { return impl->isEmpty(); }
         static void Swap(Impl& impl, Impl& other) { impl.swap(other); }
@@ skipping 50 matching lines @@
         static void DisposeWeak(const v8::WeakCallbackInfo<WeakCallbackDataType>&);
     };
 
     v8::Isolate* m_isolate;
     typename PersistentValueMapTraits::MapType m_map;
 };
 
 } // namespace blink
 
 #endif // DOMWrapperMap_h