Update to current webrtc library

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

webrtc/rtc_base/thread_checker.h

@@ -0,0 +1,27 @@
+/*
+ *  Copyright (c) 2014 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.
+ */
+
+// Borrowed from Chromium's src/base/threading/thread_checker.h.
+
+#ifndef RTC_BASE_THREAD_CHECKER_H_
+#define RTC_BASE_THREAD_CHECKER_H_
+
+#include "rtc_base/deprecation.h"
+#include "rtc_base/synchronization/sequence_checker.h"
+
+namespace rtc {
+// TODO(srte): Replace usages of this with SequenceChecker.
+class ThreadChecker : public webrtc::SequenceChecker {
+ public:
+  RTC_DEPRECATED bool CalledOnValidThread() const { return IsCurrent(); }
+  RTC_DEPRECATED void DetachFromThread() { Detach(); }
+};
+}  // namespace rtc
+#endif  // RTC_BASE_THREAD_CHECKER_H_