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/yield_policy.h

@@ -0,0 +1,38 @@
+/*
+ *  Copyright 2019 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_SYNCHRONIZATION_YIELD_POLICY_H_
+#define RTC_BASE_SYNCHRONIZATION_YIELD_POLICY_H_
+
+namespace rtc {
+class YieldInterface {
+ public:
+  virtual ~YieldInterface() = default;
+  virtual void YieldExecution() = 0;
+};
+
+// Sets the current thread-local yield policy while it's in scope and reverts
+// to the previous policy when it leaves the scope.
+class ScopedYieldPolicy final {
+ public:
+  explicit ScopedYieldPolicy(YieldInterface* policy);
+  ScopedYieldPolicy(const ScopedYieldPolicy&) = delete;
+  ScopedYieldPolicy& operator=(const ScopedYieldPolicy&) = delete;
+  ~ScopedYieldPolicy();
+  // Will yield as specified by the currently active thread-local yield policy
+  // (which by default is a no-op).
+  static void YieldExecution();
+
+ private:
+  YieldInterface* const previous_;
+};
+
+}  // namespace rtc
+
+#endif  // RTC_BASE_SYNCHRONIZATION_YIELD_POLICY_H_