Introduce infrastructure for tracing ScriptWrappables.

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

Index: third_party/WebKit/Source/bindings/core/v8/DOMWrapperMap.h
diff --git a/third_party/WebKit/Source/bindings/core/v8/DOMWrapperMap.h b/third_party/WebKit/Source/bindings/core/v8/DOMWrapperMap.h
index 33da4180d18c9081f312c987c3e2c7627cffd726..ff044644aeed2bf695240b215e20289c82609c6d 100644
--- a/third_party/WebKit/Source/bindings/core/v8/DOMWrapperMap.h
+++ b/third_party/WebKit/Source/bindings/core/v8/DOMWrapperMap.h
@@ -93,6 +93,11 @@ public:
         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);