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

chrome/browser/ui/cocoa/panels/panel_utils_cocoa.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/panels/panel_utils_cocoa.h"
 
 namespace cocoa_utils {
 
 NSRect ConvertRectToCocoaCoordinates(const gfx::Rect& bounds) {
   // Flip coordinates based on the primary screen.
-  NSScreen* screen = [[NSScreen screens] objectAtIndex:0];
+  NSScreen* screen = [[NSScreen screens] firstObject];
 
   return NSMakeRect(
       bounds.x(), NSHeight([screen frame]) - bounds.height() - bounds.y(),
       bounds.width(), bounds.height());
 }
 
 gfx::Rect ConvertRectFromCocoaCoordinates(NSRect bounds) {
   // Flip coordinates based on the primary screen.
-  NSScreen* screen = [[NSScreen screens] objectAtIndex:0];
+  NSScreen* screen = [[NSScreen screens] firstObject];
 
   return gfx::Rect(
       NSMinX(bounds), NSHeight([screen frame]) - NSMaxY(bounds),
       NSWidth(bounds), NSHeight(bounds));
 }
 
 NSPoint ConvertPointToCocoaCoordinates(const gfx::Point& point) {
   // Flip coordinates based on the primary screen.
-  NSScreen* screen = [[NSScreen screens] objectAtIndex:0];
+  NSScreen* screen = [[NSScreen screens] firstObject];
 
   return NSMakePoint(point.x(), NSHeight([screen frame]) - point.y());
 }
 
 gfx::Point ConvertPointFromCocoaCoordinates(NSPoint point) {
   // Flip coordinates based on the primary screen.
-  NSScreen* screen = [[NSScreen screens] objectAtIndex:0];
+  NSScreen* screen = [[NSScreen screens] firstObject];
 
   return gfx::Point(point.x, NSHeight([screen frame]) - point.y);
 }
 
 }  // namespace cocoa_utils