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

content/public/test/content_browser_test_utils_mac.mm

 // Copyright (c) 2012 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 "content/public/test/content_browser_test_utils.h"
 
 #include <Carbon/Carbon.h>
 #import <Cocoa/Cocoa.h>
 
 #include "ui/gfx/geometry/rect.h"
 
 namespace content {
 
 void SetWindowBounds(gfx::NativeWindow window, const gfx::Rect& bounds) {
   NSRect new_bounds = NSRectFromCGRect(bounds.ToCGRect());
   if ([[NSScreen screens] count] > 0) {
     new_bounds.origin.y =
-        [[[NSScreen screens] objectAtIndex:0] frame].size.height -
+        [[[NSScreen screens] firstObject] frame].size.height -
         new_bounds.origin.y - new_bounds.size.height;
   }
 
   [window setFrame:new_bounds display:NO];
 }
 
 }  // namespace content