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

chrome/browser/ui/cocoa/presentation_mode_controller.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.
 
 #import "chrome/browser/ui/cocoa/presentation_mode_controller.h"
 
 #include <algorithm>
 
 #include "base/command_line.h"
 #import "base/mac/mac_util.h"
@@ skipping 517 matching lines @@
   if (![[NSScreen class]
           respondsToSelector:@selector(screensHaveSeparateSpaces)])
     return [self isWindowOnPrimaryScreen];
 
   BOOL eachScreenShouldHaveMenuBar = [NSScreen screensHaveSeparateSpaces];
   return eachScreenShouldHaveMenuBar ?: [self isWindowOnPrimaryScreen];
 }
 
 - (BOOL)isWindowOnPrimaryScreen {
   NSScreen* screen = [[browserController_ window] screen];
-  NSScreen* primaryScreen = [[NSScreen screens] objectAtIndex:0];
+  NSScreen* primaryScreen = [[NSScreen screens] firstObject];
   return (screen == primaryScreen);
 }
 
 - (base::mac::FullScreenMode)desiredSystemFullscreenMode {
   if ([self shouldShowMenubarInImmersiveFullscreen])
     return base::mac::kFullScreenModeHideDock;
   return base::mac::kFullScreenModeHideAll;
 }
 
 - (void)changeOverlayToFraction:(CGFloat)fraction
@@ skipping 197 matching lines @@
   [self updateMenuBarAndDockVisibility];
 
   // TODO(rohitrao): Remove the Exit Fullscreen button.  http://crbug.com/35956
 }
 
 - (BOOL)shouldShowMenubarInImmersiveFullscreen {
   return [self doesScreenHaveMenuBar] && toolbarFraction_ > 0.99;
 }
 
 @end