Introduce phantom weak handles in the API and use them internally for debug info

test/cctest/test-api.cc

Index: test/cctest/test-api.cc
diff --git a/test/cctest/test-api.cc b/test/cctest/test-api.cc
index b458418f43a66147a814dbd669ec58b1043f88c4..984f985e5ff3d2cb8edb846eb5c8bf5d9acc6fd5 100644
--- a/test/cctest/test-api.cc
+++ b/test/cctest/test-api.cc
@@ -7479,8 +7479,8 @@ THREADED_TEST(IndependentWeakHandle) {
 
   object_a.flag = false;
   object_b.flag = false;
-  object_a.handle.SetWeak(&object_a, &DisposeAndSetFlag);
-  object_b.handle.SetWeak(&object_b, &DisposeAndSetFlag);
+  object_a.handle.SetPhantom(&object_a, &DisposeAndSetFlag);
+  object_b.handle.SetPhantom(&object_b, &DisposeAndSetFlag);
   CHECK(!object_b.handle.IsIndependent());
   object_a.handle.MarkIndependent();
   object_b.handle.MarkIndependent();