Re-add a (dummy) webrtc/base/buffer.cc to hopefully unbreak the Chromium build

Remove this file once Chromium doesn't need it anymore.

TBR=tommi@webrtc.org

BUG=webrtc:5845

Review URL: https://codereview.webrtc.org/1928633006 .

Cr-Commit-Position: refs/heads/master@{#12568}
diff --git a/webrtc/base/buffer.cc b/webrtc/base/buffer.cc
new file mode 100644
index 0000000..79e48bd
--- /dev/null
+++ b/webrtc/base/buffer.cc
@@ -0,0 +1,12 @@
+/*
+ *  Copyright 2016 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.
+ */
+
+// This is a dummy file, added because something in the Chromium build claims
+// to need it. We should fix that, and then remove it (bug 5845).