[runtime] Fix integer indexed property handling

test/mjsunit/regress/regress-integer-indexed-element.js

+// Copyright 2016 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 = {__proto__:new Int32Array(100)};
+Object.prototype[1.3] = 10;
+assertEquals(10, o[1.3]);
+
+var o = new Int32Array(100);
+var o2 = new Int32Array(200);
+o.__proto__ = o2;
+assertEquals(undefined, Reflect.get(o, 1.3, o2));