Create a reader-writer lock implementaiton in base.

base/synchronization/rw_lock_nacl.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/synchronization/rw_lock.h"
+
+#include "base/logging.h"
+
+namespace base {
+
+RWLock::RWLock() {}
+
+RWLock::~RWLock() {
+  DCHECK_EQ(0u, readers_);
+}
+
+void RWLock::ReadAcquire() {
+  AutoLock l(native_handle_);

[danakj, 2016/05/19 23:01:37]

nit: name var "hold". ditto elsewhere

[Anand Mistry (off Chromium), 2016/05/20 04:06:38]

Done.

+  readers_++;
+  if (readers_ == 1)
+    writer_lock_.Acquire();
+}
+
+void RWLock::ReadRelease() {
+  AutoLock l(native_handle_);
+  readers_--;
+  if (readers_ == 0)
+    writer_lock_.Release();
+}
+
+void RWLock::WriteAcquire() {
+  writer_lock_.Acquire();
+}
+
+void RWLock::WriteRelease() {
+  writer_lock_.Release();
+}
+
+}  // namespace base