Make object groups and implicit references a bit more lightweight.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index b0d287ded90dab25a7fa91d289b3099852547d29..056c0be9cd0c0caccfdaf601ceace094dde6689c 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -4473,7 +4473,7 @@ void V8::AddImplicitReferences(Persistent<Object> parent,
   if (IsDeadCheck(isolate, "v8::V8::AddImplicitReferences()")) return;
   STATIC_ASSERT(sizeof(Persistent<Value>) == sizeof(i::Object**));
   isolate->global_handles()->AddImplicitReferences(
-      *Utils::OpenHandle(*parent),
+      i::Handle<i::HeapObject>::cast(Utils::OpenHandle(*parent)).location(),
       reinterpret_cast<i::Object***>(children), length);
 }