Turn ThreadPriority enum into an enum class.

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;
 }
 
 ThreadPriority NiceValueToThreadPriority(int nice_value) {
   for (const ThreadPriorityToNiceValuePair& pair :
        kThreadPriorityToNiceValueMap) {
     if (pair.nice_value == nice_value)
       return pair.priority;
   }
   NOTREACHED() << "Unknown nice value";
-  return kThreadPriority_Normal;
+  return ThreadPriority::NORMAL;
 }
 
 }  // namespace internal
 
 }  // namespace base