Mac: Use [NSArray firstObject] for [NSScreen screens]

ui/message_center/cocoa/popup_collection.mm

 // Copyright (c) 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.
 
 #import "ui/message_center/cocoa/popup_collection.h"
 
 #import "ui/message_center/cocoa/notification_controller.h"
 #import "ui/message_center/cocoa/popup_controller.h"
 #include "ui/message_center/message_center.h"
 #include "ui/message_center/message_center_observer.h"
@@ skipping 132 matching lines @@
 - (void)setAnimationEndedCallback:
     (message_center::AnimationEndedCallback)callback {
   testingAnimationEndedCallback_.reset(Block_copy(callback));
 }
 
 // Private /////////////////////////////////////////////////////////////////////
 
 - (NSRect)screenFrame {
   if (!NSIsEmptyRect(testingScreenFrame_))
     return testingScreenFrame_;
-  return [[[NSScreen screens] objectAtIndex:0] visibleFrame];
+  return [[[NSScreen screens] firstObject] visibleFrame];
 }
 
 - (BOOL)addNotification:(const message_center::Notification*)notification {
   // Wait till all existing animations end.
   if ([self isAnimating])
     return NO;
 
   // The popup is owned by itself. It will be released at close.
   MCPopupController* popup =
       [[MCPopupController alloc] initWithNotification:notification
@@ skipping 225 matching lines @@
   }
 
   pendingUpdateNotificationIDs_.clear();
 
   // Start re-layout of all notifications, so that it readjusts the Y origin of
   // all updated popups and any popups that come below them.
   [self layoutNotifications];
 }
 
 @end