Use correct LookupIterator in CallSite::GetMethodName.

test/mjsunit/regress/regress-crbug-505370.js

+// Copyright 2015 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.
+
+var o = {
+  get 0() { reference_error;  },
+  get length() { return 1; }
+};
+
+var method_name;
+
+try {
+  o[0];
+} catch (e) {
+  thrown = true;
+  Error.prepareStackTrace = function(exception, frames) { return frames; };
+  var frames = e.stack;
+  Error.prepareStackTrace = undefined;
+  method_name = frames[0].getMethodName();
+}
+
+assertEquals("0", method_name);