Return undefined from RegExp.prototype.compile

This was changed to match Annex B.2.5.1 of ES2015 and Firefox in
https://chromium.googlesource.com/v8/v8/+/469d9bfa, but website
breakage was seen in M49 Beta. JSC still returns undefined here.

BUG=chromium:585775
LOG=y
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_chromium_rel_ng;tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1714903004

Cr-Commit-Position: refs/heads/master@{#34172}

src/js/regexp.js

@@ -112,7 +112,10 @@ function RegExpCompileJS(pattern, flags) {
     pattern = REGEXP_SOURCE(pattern);
   }
 
-  return RegExpInitialize(this, pattern, flags);
+  RegExpInitialize(this, pattern, flags);
+
+  // Return undefined for compatibility with JSC.
+  // See http://crbug.com/585775 for web compat details.
 }
 
 

test/mjsunit/regress/regress-585775.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 pattern = /foo/;
+assertEquals(undefined, pattern.compile(pattern));