[runtime] Introduce dedicated JSBoundFunction to represent bound functions.

src/json-stringifier.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_JSON_STRINGIFIER_H_
 #define V8_JSON_STRINGIFIER_H_
 
 #include "src/conversions.h"
 #include "src/lookup.h"
 #include "src/messages.h"
@@ skipping 319 matching lines @@
         default:
           return UNCHANGED;
       }
     case JS_ARRAY_TYPE:
       if (object->IsAccessCheckNeeded()) break;
       if (deferred_string_key) SerializeDeferredKey(comma, key);
       return SerializeJSArray(Handle<JSArray>::cast(object));
     case JS_VALUE_TYPE:
       if (deferred_string_key) SerializeDeferredKey(comma, key);
       return SerializeJSValue(Handle<JSValue>::cast(object));
-    case JS_FUNCTION_TYPE:
-      return UNCHANGED;
     default:
       if (object->IsString()) {
         if (deferred_string_key) SerializeDeferredKey(comma, key);
         SerializeString(Handle<String>::cast(object));
         return SUCCESS;
       } else if (object->IsJSObject()) {
+        if (object->IsCallable()) return UNCHANGED;
         // Go to slow path for global proxy and objects requiring access checks.
         if (object->IsAccessCheckNeeded() || object->IsJSGlobalProxy()) break;
         if (deferred_string_key) SerializeDeferredKey(comma, key);
         return SerializeJSObject(Handle<JSObject>::cast(object));
       }
   }
 
   return SerializeGeneric(object, key, comma, deferred_string_key);
 }
 
@@ skipping 322 matching lines @@
     } else {
       SerializeString_<uc16, uc16>(object);
     }
   }
 }
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_JSON_STRINGIFIER_H_