Introduces rtc_base/synchronization/mutex.h.

This change introduces a new non-reentrant mutex to WebRTC. It
enables eventual migration to Abseil's mutex.

The mutex types supportable by webrtc::Mutex are

- absl::Mutex
- CriticalSection (Windows only)
- pthread_mutex (POSIX only)

In addition to introducing the mutexes, the CL also changes
PacketBuffer to use the new mutex instead of rtc::CriticalSection.

The method of yielding from critical_section.cc was given a
mini-cleanup and YieldCurrentThread() was added to
rtc_base/synchronization/yield.h/cc.

Additionally, google_benchmark benchmarks for the mutexes were added
(test courtesy of danilchap@), and some results from a pthread/Abseil
shootout were added showing Abseil has the advantage in higher
contention.

Bug: webrtc:11567, webrtc:11634
Change-Id: Iaec324ccb32ec3851bf6db3fd290f5ea5dee4c81
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/176230
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31443}

rtc_base/synchronization/yield.cc

@@ -0,0 +1,36 @@
+/*
+ *  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/yield.h"
+
+#if defined(WEBRTC_WIN)
+#include <windows.h>
+#else
+#include <sched.h>
+#include <time.h>
+#endif
+
+namespace webrtc {
+
+void YieldCurrentThread() {
+  // TODO(bugs.webrtc.org/11634): use dedicated OS functionality instead of
+  // sleep for yielding.
+#if defined(WEBRTC_WIN)
+  ::Sleep(0);
+#elif defined(WEBRTC_MAC) && defined(RTC_USE_NATIVE_MUTEX_ON_MAC) && \
+    !RTC_USE_NATIVE_MUTEX_ON_MAC
+  sched_yield();
+#else
+  static const struct timespec ts_null = {0};
+  nanosleep(&ts_null, nullptr);
+#endif
+}
+
+}  // namespace webrtc