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

content/browser/renderer_host/input/web_input_event_builders_mac.mm

 // Copyright 2015 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.
 
 /*
  * Copyright (C) 2004, 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2006-2009 Google Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ skipping 484 matching lines @@
 
 void SetWebEventLocationFromEventInView(blink::WebMouseEvent* result,
                                         NSEvent* event,
                                         NSView* view) {
   NSPoint window_local = [event locationInWindow];
 
   NSPoint screen_local = [[view window] convertBaseToScreen:window_local];
   result->globalX = screen_local.x;
   // Flip y.
   NSScreen* primary_screen = ([[NSScreen screens] count] > 0)
-                                 ? [[NSScreen screens] objectAtIndex:0]
+                                 ? [[NSScreen screens] firstObject]
                                  : nil;
   if (primary_screen)
     result->globalY = [primary_screen frame].size.height - screen_local.y;
   else
     result->globalY = screen_local.y;
 
   NSPoint content_local = [view convertPoint:window_local fromView:nil];
   result->x = content_local.x;
   result->y = [view frame].size.height - content_local.y;  // Flip y.
 
@@ skipping 402 matching lines @@
       break;
     default:
       NOTIMPLEMENTED();
       result.type = blink::WebInputEvent::Undefined;
   }
 
   return result;
 }
 
 }  // namespace content