Revert of Reland #1: "base: Implement GetCurrentThreadPriority."

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 4db5a36f8a37b7768e23b4f5f1193072dbb06b8f..e743044ec1a0845986ee7d49d6befa95b3e44431 100644
--- a/base/threading/platform_thread_mac.mm
+++ b/base/threading/platform_thread_mac.mm
@@ -205,9 +205,10 @@
     case ThreadPriority::DISPLAY:
     case ThreadPriority::REALTIME_AUDIO:
       return thread_priority;
-  }
-  NOTREACHED() << "Unknown priority.";
-  return ThreadPriority::NORMAL;
+    default:
+      NOTREACHED() << "Unknown priority.";
+      return ThreadPriority::NORMAL;
+  }
 }
 
 size_t GetDefaultThreadStackSize(const pthread_attr_t& attributes) {