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/system_wrappers/include/denormal_disabler.h

@@ -0,0 +1,56 @@
+/*
+ *  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.
+ */
+
+#ifndef SYSTEM_WRAPPERS_INCLUDE_DENORMAL_DISABLER_H_
+#define SYSTEM_WRAPPERS_INCLUDE_DENORMAL_DISABLER_H_
+
+#include "rtc_base/system/arch.h"
+
+namespace webrtc {
+
+// Activates the hardware (HW) way to flush denormals (see [1]) to zero as they
+// can very seriously impact performance. At destruction time restores the
+// denormals handling state read by the ctor; hence, supports nested calls.
+// Equals a no-op if the architecture is not x86 or ARM or if the compiler is
+// not CLANG.
+// [1] https://en.wikipedia.org/wiki/Denormal_number
+//
+// Example usage:
+//
+// void Foo() {
+//   DenormalDisabler d;
+//   ...
+// }
+class DenormalDisabler {
+ public:
+  // Ctor. If architecture and compiler are supported, stores the HW settings
+  // for denormals, disables denormals and sets `disabling_activated_` to true.
+  // Otherwise, only sets `disabling_activated_` to false.
+  DenormalDisabler();
+  // Ctor. Same as above, but also requires `enabled` to be true to disable
+  // denormals.
+  explicit DenormalDisabler(bool enabled);
+  DenormalDisabler(const DenormalDisabler&) = delete;
+  DenormalDisabler& operator=(const DenormalDisabler&) = delete;
+  // Dtor. If `disabling_activated_` is true, restores the denormals HW settings
+  // read by the ctor before denormals were disabled. Otherwise it's a no-op.
+  ~DenormalDisabler();
+
+  // Returns true if architecture and compiler are supported.
+  static bool IsSupported();
+
+ private:
+  const int status_word_;
+  const bool disabling_activated_;
+};
+
+}  // namespace webrtc
+
+#endif  // SYSTEM_WRAPPERS_INCLUDE_DENORMAL_DISABLER_H_