Land test case for RegExp.source.

BUG=chromium:447561
LOG=N
TBR=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/878033003

Cr-Commit-Position: refs/heads/master@{#26297}

test/mjsunit/regress/regress-447561.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.
+
+__proto__ = /foo/gi;
+assertEquals("foo", source);
+assertTrue(global);
+assertTrue(ignoreCase);
+assertFalse(multiline);
+assertEquals(0, lastIndex);