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

chrome/browser/ui/app_list/app_list_service_mac.mm

 // 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.
 
 #import "chrome/browser/ui/app_list/app_list_service_mac.h"
 
 #include <ApplicationServices/ApplicationServices.h>
 #import <Cocoa/Cocoa.h>
 
 #include "base/bind.h"
@@ skipping 232 matching lines @@
         screen->GetDisplayNearestWindow(key_view) :
         screen->GetPrimaryDisplay();
   } else {
     cursor = screen->GetCursorScreenPoint();
     display = screen->GetDisplayNearestPoint(cursor);
   }
 
   const NSSize ns_window_size = [window frame].size;
   gfx::Size window_size(ns_window_size.width, ns_window_size.height);
   int primary_display_height =
-      NSMaxY([[[NSScreen screens] objectAtIndex:0] frame]);
+      NSMaxY([[[NSScreen screens] firstObject] frame]);
   AppListServiceMac::FindAnchorPoint(window_size,
                                      display,
                                      primary_display_height,
                                      cursor_is_visible,
                                      cursor,
                                      target_origin,
                                      start_origin);
 }
 
 AppListServiceMac* GetActiveInstance() {
@@ skipping 314 matching lines @@
 
 - (void)animationDidEnd:(NSAnimation*)animation {
   content::BrowserThread::PostTask(
       content::BrowserThread::UI,
       FROM_HERE,
       base::Bind(&AppListServiceMac::WindowAnimationDidEnd,
                  base::Unretained(GetActiveInstance())));
 }
 
 @end