[literals] Document that literals with simple getters cause dict-properties

Drive-by-fix: increase coverage in object literals test.

Change-Id: Iccfdf35b29229f63b3e8c1d961bee56ee03da688
Reviewed-on: https://chromium-review.googlesource.com/751661Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49105}

test/mjsunit/object-literal-modified-object-prototype.js

+// Copyright 2017 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.
+
+(function TestModifedPrototypeInObjectLiteral() {
+  // The prototype chain should not be used if the definition
+  // happens in the object literal.
+
+  Object.defineProperty(Object.prototype, 'c', {
+    get: function () {
+      return 21;
+    },
+    set: function () {
+    }
+  });
+
+  var o = {};
+  o.c = 7;
+  assertEquals(21, o.c);
+
+  var l = {c: 7};
+  assertEquals(7, l.c);
+
+  delete Object.prototype.c;
+})();