base: Support setting thread priorities generically.

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 48041a06a4f0ba8b3e44b1fc3e19ff748a46942c..79b3b9176b4139af0eec98bb1dbd27a3bd4a8526 100644
--- a/base/threading/platform_thread_mac.mm
+++ b/base/threading/platform_thread_mac.mm
@@ -161,6 +161,7 @@ void SetPriorityRealtimeAudio(mach_port_t mach_thread_id) {
 
 // static
 void PlatformThread::SetThreadPriority(PlatformThreadHandle handle,
+                                       PlatformThreadId,
                                        ThreadPriority priority) {
   // Convert from pthread_t to mach thread identifier.
   mach_port_t mach_thread_id = pthread_mach_thread_np(handle);
@@ -172,6 +173,9 @@ void PlatformThread::SetThreadPriority(PlatformThreadHandle handle,
     case kThreadPriority_RealtimeAudio:
       SetPriorityRealtimeAudio(mach_thread_id);
       break;
+    default:
+      NOTREACHED() << "Unknown priority.";
+      break;
   }
 }