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

content/browser/renderer_host/render_widget_host_view_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 "content/browser/renderer_host/render_widget_host_view_mac.h"
 
 #import <objc/runtime.h>
 #include <OpenGL/gl.h>
 #include <QuartzCore/QuartzCore.h>
 
@@ skipping 336 matching lines @@
 // on NSApplicationDidChangeScreenParametersNotification. Only one observer
 // is necessary.
 bool g_screen_info_up_to_date = false;
 
 float FlipYFromRectToScreen(float y, float rect_height) {
   TRACE_EVENT0("browser", "FlipYFromRectToScreen");
   static CGFloat screen_zero_height = 0;
   if (!g_screen_info_up_to_date) {
     if ([[NSScreen screens] count] > 0) {
       screen_zero_height =
-          [[[NSScreen screens] objectAtIndex:0] frame].size.height;
+          [[[NSScreen screens] firstObject] frame].size.height;
       g_screen_info_up_to_date = true;
     } else {
       return y;
     }
   }
   return screen_zero_height - y - rect_height;
 }
 
 // Adjusts an NSRect in Cocoa screen coordinates to have an origin in the upper
 // left of the primary screen (Carbon coordinates), and stuffs it into a
@@ skipping 3112 matching lines @@
 
 // "-webkit-app-region: drag | no-drag" is implemented on Mac by excluding
 // regions that are not draggable. (See ControlRegionView in
 // native_app_window_cocoa.mm). This requires the render host view to be
 // draggable by default.
 - (BOOL)mouseDownCanMoveWindow {
   return YES;
 }
 
 @end