LibJS: Fix this_value in native setters and getters

This fixes getting values from double proxies:
var p = new Proxy(new Proxy([], {}), {});
p.length

Userland/Libraries/LibJS/Runtime/Object.cpp

@@ -278,7 +278,7 @@ Value Object::get_own_property(const PropertyName& property_name, Value receiver
  if (value_here.is_accessor())
  return value_here.as_accessor().call_getter(receiver);
  if (value_here.is_native_property())
- return call_native_property_getter(value_here.as_native_property(), receiver);
+ return call_native_property_getter(value_here.as_native_property(), this);
  }
  return value_here;
 }
@@ -937,9 +937,7 @@ bool Object::put_by_index(u32 property_index, Value value)
  return true;
  }
  if (value_here.value.is_native_property()) {
- // FIXME: Why doesn't put_by_index() receive the receiver value from put()?!
- auto receiver = this;
- call_native_property_setter(value_here.value.as_native_property(), receiver, value);
+ call_native_property_setter(value_here.value.as_native_property(), this, value);
  return true;
  }
  }
@@ -976,7 +974,7 @@ bool Object::put(const PropertyName& property_name, Value value, Value receiver)
  return true;
  }
  if (value_here.is_native_property()) {
- call_native_property_setter(value_here.as_native_property(), receiver, value);
+ call_native_property_setter(value_here.as_native_property(), this, value);
  return true;
  }
  }