Bump to WebRTC M120 release

Some API deprecation -- ExperimentalAgc and ExperimentalNs are gone.
We're continuing to carry iSAC even though it's gone upstream, but maybe
we'll want to drop that soon.

webrtc/rtc_base/containers/flat_tree.cc

@@ -0,0 +1,19 @@
+/*
+ *  Copyright (c) 2021 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 implementation is borrowed from Chromium.
+
+#include "rtc_base/containers/flat_tree.h"
+
+namespace webrtc {
+
+sorted_unique_t sorted_unique;
+
+}  // namespace webrtc