Update to current webrtc library

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

webrtc/rtc_base/synchronization/mutex.cc

@@ -0,0 +1,39 @@
+/*
+ *  Copyright 2020 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.
+ */
+
+#include "rtc_base/synchronization/mutex.h"
+
+#include "rtc_base/checks.h"
+#include "rtc_base/synchronization/yield.h"
+
+namespace webrtc {
+
+#if !defined(WEBRTC_ABSL_MUTEX)
+void GlobalMutex::Lock() {
+  while (mutex_locked_.exchange(1)) {
+    YieldCurrentThread();
+  }
+}
+
+void GlobalMutex::Unlock() {
+  int old = mutex_locked_.exchange(0);
+  RTC_DCHECK_EQ(old, 1) << "Unlock called without calling Lock first";
+}
+
+GlobalMutexLock::GlobalMutexLock(GlobalMutex* mutex) : mutex_(mutex) {
+  mutex_->Lock();
+}
+
+GlobalMutexLock::~GlobalMutexLock() {
+  mutex_->Unlock();
+}
+#endif  // #if !defined(WEBRTC_ABSL_MUTEX)
+
+}  // namespace webrtc