Update comment in startup_metric_utils.

components/startup_metric_utils/browser/startup_metric_utils.cc

 // Copyright 2013 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 "components/startup_metric_utils/browser/startup_metric_utils.h"
 
 #include <stddef.h>
 
 #include "base/containers/hash_tables.h"
 #include "base/environment.h"
@@ skipping 251 matching lines @@
 // also the time taken to synchronously resolve base::Time::Now() and
 // base::TimeTicks::Now() at play, but in practice it is pretty much instant
 // compared to multi-seconds startup timings.
 base::TimeTicks StartupTimeToTimeTicks(const base::Time& time) {
   // First get a base which represents the same point in time in both units.
   // Bump the priority of this thread while doing this as the wall clock time it
   // takes to resolve these two calls affects the precision of this method and
   // 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;
 
   base::ThreadPriority previous_priority = base::ThreadPriority::NORMAL;
   if (!statics_initialized) {
     previous_priority = base::PlatformThread::GetCurrentThreadPriority();
     base::PlatformThread::SetCurrentThreadPriority(
         base::ThreadPriority::DISPLAY);
   }
 #endif
@@ skipping 418 matching lines @@
 
 base::TimeTicks MainEntryPointTicks() {
   return g_browser_main_entry_point_ticks.Get();
 }
 
 StartupTemperature GetStartupTemperature() {
   return g_startup_temperature;
 }
 
 }  // namespace startup_metric_utils