[runtime] Introduce dedicated JSBoundFunction to represent bound functions.

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 12e99a08474feee263357cfc58d938b2fbc258a2..9f0c9052e1a2630ebda39f35927ed1cada56a67c 100644
--- a/test/cctest/test-api.cc
+++ b/test/cctest/test-api.cc
@@ -11670,7 +11670,7 @@ THREADED_TEST(Regress567998) {
   CHECK(
       env->Global()->Set(env.local(), v8_str("undetectable"), obj).FromJust());
 
-  ExpectString("undetectable.toString()", "[object Object]");
+  ExpectString("undetectable.toString()", "[object Function]");
   ExpectString("typeof undetectable", "undefined");
   ExpectString("typeof(undetectable)", "undefined");
   ExpectBoolean("typeof undetectable == 'undefined'", true);