Deduplicate PlatformThread's POSIX implementations of SetThreadPriority.

base/threading/platform_thread_internal_posix.cc

+// Copyright 2015 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/threading/platform_thread_internal_posix.h"
+
+#include "base/logging.h"
+
+namespace base {
+
+namespace internal {
+
+int ThreadPriorityToNiceValue(ThreadPriority priority) {
+  for (const ThreadPriorityToNiceValuePair& pair :
+       kThreadPriorityToNiceValueMap) {
+    if (pair.priority == priority)
+      return pair.nice_value;
+  }
+  NOTREACHED() << "Unknown ThreadPriority";
+  return 0;
+}
+
+}  // namespace internal
+
+}  // namespace base