Object.observe: avoid accessing acceptList properties more than once

test/mjsunit/regress/regress-3315.js

+// Copyright 2014 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 indexZeroCallCount = 0;
+var indexOneCallCount = 0;
+var lengthCallCount = 0;
+var acceptList = {
+  get 0() {
+    indexZeroCallCount++;
+    return 'foo';
+  },
+  get 1() {
+    indexOneCallCount++;
+    return 'bar';
+  },
+  get length() {
+    lengthCallCount++;
+    return 1;
+  }
+};
+
+Object.observe({}, function(){}, acceptList);
+assertEquals(1, lengthCallCount);
+assertEquals(1, indexZeroCallCount);
+assertEquals(0, indexOneCallCount);