Turn ThreadPriority enum into an enum class.

base/threading/platform_thread_mac.mm

Index: base/threading/platform_thread_mac.mm
diff --git a/base/threading/platform_thread_mac.mm b/base/threading/platform_thread_mac.mm
index 59345b005db13c26d2e80e5637d54d8c05ece90b..fd40d7952c3bc9d1bf7c049629fce7a74b135373 100644
--- a/base/threading/platform_thread_mac.mm
+++ b/base/threading/platform_thread_mac.mm
@@ -161,10 +161,10 @@ void PlatformThread::SetThreadPriority(PlatformThreadHandle handle,
   mach_port_t mach_thread_id = pthread_mach_thread_np(handle.handle_);
 
   switch (priority) {
-    case kThreadPriority_Normal:
+    case ThreadPriority::NORMAL:
       SetPriorityNormal(mach_thread_id);
       break;
-    case kThreadPriority_RealtimeAudio:
+    case ThreadPriority::REALTIME_AUDIO:
       SetPriorityRealtimeAudio(mach_thread_id);
       break;
     default:
@@ -176,7 +176,7 @@ void PlatformThread::SetThreadPriority(PlatformThreadHandle handle,
 // static
 ThreadPriority PlatformThread::GetThreadPriority(PlatformThreadHandle handle) {
   NOTIMPLEMENTED();
-  return kThreadPriority_Normal;
+  return ThreadPriority::NORMAL;
 }
 
 size_t GetDefaultThreadStackSize(const pthread_attr_t& attributes) {