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

ui/snapshot/snapshot_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 "ui/snapshot/snapshot.h"
 
 #import <Cocoa/Cocoa.h>
 
 #include "base/callback.h"
 #include "base/logging.h"
 #include "base/mac/scoped_cftyperef.h"
 #include "base/mac/scoped_nsobject.h"
 #include "base/task_runner.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/gfx/image/image.h"
 
 namespace ui {
 
 bool GrabViewSnapshot(gfx::NativeView view,
                       std::vector<unsigned char>* png_representation,
                       const gfx::Rect& snapshot_bounds) {
   NSWindow* window = [view window];
-  NSScreen* screen = [[NSScreen screens] objectAtIndex:0];
+  NSScreen* screen = [[NSScreen screens] firstObject];
   gfx::Rect screen_bounds = gfx::Rect(NSRectToCGRect([screen frame]));
 
 
   // Get the view bounds relative to the screen
   NSRect frame = [view convertRect:[view bounds] toView:nil];
   frame.origin = [window convertBaseToScreen:frame.origin];
 
   gfx::Rect view_bounds = gfx::Rect(NSRectToCGRect(frame));
 
   // Flip window coordinates based on the primary screen.
@@ skipping 61 matching lines @@
 void GrabWindowSnapshotAsync(
     gfx::NativeWindow window,
     const gfx::Rect& source_rect,
     scoped_refptr<base::TaskRunner> background_task_runner,
     const GrabWindowSnapshotAsyncPNGCallback& callback) {
   return GrabViewSnapshotAsync([[window contentView] superview], source_rect,
       background_task_runner, callback);
 }
 
 }  // namespace ui