Update to current webrtc library

This is from the upstream library commit id
3326535126e435f1ba647885ce43a8f0f3d317eb, corresponding to Chromium
88.0.4290.1.

webrtc/rtc_base/system/unused.h

@@ -0,0 +1,39 @@
+/*
+ *  Copyright (c) 2018 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 RTC_BASE_SYSTEM_UNUSED_H_
+#define RTC_BASE_SYSTEM_UNUSED_H_
+
+// Annotate a function indicating the caller must examine the return value.
+// Use like:
+//   int foo() RTC_WARN_UNUSED_RESULT;
+// To explicitly ignore a result, cast to void.
+// TODO(kwiberg): Remove when we can use [[nodiscard]] from C++17.
+#if defined(__clang__)
+#define RTC_WARN_UNUSED_RESULT __attribute__((__warn_unused_result__))
+#elif defined(__GNUC__)
+// gcc has a __warn_unused_result__ attribute, but you can't quiet it by
+// casting to void, so we don't use it.
+#define RTC_WARN_UNUSED_RESULT
+#else
+#define RTC_WARN_UNUSED_RESULT
+#endif
+
+// Prevent the compiler from warning about an unused variable. For example:
+//   int result = DoSomething();
+//   assert(result == 17);
+//   RTC_UNUSED(result);
+// Note: In most cases it is better to remove the unused variable rather than
+// suppressing the compiler warning.
+#ifndef RTC_UNUSED
+#define RTC_UNUSED(x) static_cast<void>(x)
+#endif  // RTC_UNUSED
+
+#endif  // RTC_BASE_SYSTEM_UNUSED_H_