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

ui/gfx/mac/coordinate_conversion.mm

 // Copyright 2014 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 "ui/gfx/mac/coordinate_conversion.h"
 
 #import <Cocoa/Cocoa.h>
 
 #include "ui/gfx/geometry/point.h"
 #include "ui/gfx/geometry/rect.h"
 
 namespace gfx {
 
 namespace {
 
 // The height of the primary display, which OSX defines as the monitor with the
 // menubar. This is always at index 0.
 CGFloat PrimaryDisplayHeight() {
-  return NSMaxY([[[NSScreen screens] objectAtIndex:0] frame]);
+  return NSMaxY([[[NSScreen screens] firstObject] frame]);
 }
 
 }  // namespace
 
 NSRect ScreenRectToNSRect(const Rect& rect) {
   return NSMakeRect(rect.x(),
                     PrimaryDisplayHeight() - rect.y() - rect.height(),
                     rect.width(),
                     rect.height());
 }
 
 Rect ScreenRectFromNSRect(const NSRect& rect) {
   return Rect(rect.origin.x,
               PrimaryDisplayHeight() - rect.origin.y - rect.size.height,
               rect.size.width, rect.size.height);
 }
 
 NSPoint ScreenPointToNSPoint(const Point& point) {
   return NSMakePoint(point.x(), PrimaryDisplayHeight() - point.y());
 }
 
 Point ScreenPointFromNSPoint(const NSPoint& point) {
   return Point(point.x, PrimaryDisplayHeight() - point.y);
 }
 
 }  // namespace gfx