Report initial_map field of JSFunction in heap snapshot.

src/heap-snapshot-generator.cc

Index: src/heap-snapshot-generator.cc
diff --git a/src/heap-snapshot-generator.cc b/src/heap-snapshot-generator.cc
index f5257e67033ca91e747f2e00c9d886bfe235d62b..1d7028e08d26c1ed3954f11e0b7e6bcf880eb9a4 100644
--- a/src/heap-snapshot-generator.cc
+++ b/src/heap-snapshot-generator.cc
@@ -999,6 +999,9 @@ void V8HeapExplorer::ExtractJSObjectReferences(
         SetPropertyReference(
             obj, entry,
             heap_->prototype_string(), js_fun->prototype());
+        SetInternalReference(
+            obj, entry, "initial_map", proto_or_map,
+            JSFunction::kPrototypeOrInitialMapOffset);
       }
     }
     SharedFunctionInfo* shared_info = js_fun->shared();