[runtime] Introduce dedicated JSBoundFunction to represent bound functions.

test/cctest/test-api.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 11652 matching lines @@
   desc->InstanceTemplate()->MarkAsUndetectable();  // undetectable
   desc->InstanceTemplate()->SetCallAsFunctionHandler(ReturnThis);  // callable
 
   Local<v8::Object> obj = desc->GetFunction(env.local())
                               .ToLocalChecked()
                               ->NewInstance(env.local())
                               .ToLocalChecked();
   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);
   ExpectBoolean("typeof undetectable == 'object'", false);
   ExpectBoolean("if (undetectable) { true; } else { false; }", false);
   ExpectBoolean("!undetectable", true);
 
   ExpectObject("true&&undetectable", obj);
   ExpectBoolean("false&&undetectable", false);
   ExpectBoolean("true||undetectable", true);
@@ skipping 12596 matching lines @@
     CHECK_NE(*object->Get(env2.local(), v8_str("values")).ToLocalChecked(),
              *object2->Get(env2.local(), v8_str("values")).ToLocalChecked());
 
     auto values2 = Local<Function>::Cast(
         object2->Get(env2.local(), v8_str("values")).ToLocalChecked());
     auto fn2 = i::Handle<i::JSFunction>::cast(v8::Utils::OpenHandle(*values2));
     auto ctx2 = v8::Utils::OpenHandle(*env2.local());
     CHECK_EQ(fn2->GetCreationContext(), *ctx2);
   }
 }