Reland "Move webrtc/{base => rtc_base} (stub headers)

Add the stub headers from https://codereview.webrtc.org/2877023002
as a separate commit. This preserves git blame history of the moved files.

BUG=webrtc:7634
NOTRY=True
TBR=kwiberg@webrtc.org

Change-Id: Ic141abf11801fbfdeea5bcdb23608696ad449013
Reviewed-on: https://chromium-review.googlesource.com/554623
Reviewed-by: Henrik Kjellander <kjellander@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18822}

webrtc/base/asyncpacketsocket.h

@@ -0,0 +1,19 @@
+/*
+ *  Copyright 2004 The WebRTC 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 in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_BASE_ASYNCPACKETSOCKET_H_
+#define WEBRTC_BASE_ASYNCPACKETSOCKET_H_
+
+
+// This header is deprecated and is just left here temporarily during
+// refactoring. See https://bugs.webrtc.org/7634 for more details.
+#include "webrtc/rtc_base/asyncpacketsocket.h"
+
+#endif  // WEBRTC_BASE_ASYNCPACKETSOCKET_H_