Update comment in startup_metric_utils.

components/startup_metric_utils/browser/startup_metric_utils.cc

Index: components/startup_metric_utils/browser/startup_metric_utils.cc
diff --git a/components/startup_metric_utils/browser/startup_metric_utils.cc b/components/startup_metric_utils/browser/startup_metric_utils.cc
index 16f41031316b25c2164d31312e976bbb019b6567..fa16252ea02483aaf77ebc587564e852368f37ba 100644
--- a/components/startup_metric_utils/browser/startup_metric_utils.cc
+++ b/components/startup_metric_utils/browser/startup_metric_utils.cc
@@ -269,8 +269,8 @@ base::TimeTicks StartupTimeToTimeTicks(const base::Time& time) {
   // bumping the priority reduces the likelihood of a context switch interfering
   // with this computation.
 
-// platform_thread_mac.mm unfortunately doesn't properly support base's
-// thread priority APIs (crbug.com/554651).
+// Enabling this logic on OS X causes a significant performance regression.
+// https://crbug.com/601270
 #if !defined(OS_MACOSX)
   static bool statics_initialized = false;