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

chrome/browser/ui/window_sizer/window_sizer_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 "chrome/browser/ui/window_sizer/window_sizer.h"
 
 #import <Cocoa/Cocoa.h>
 
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/browser_finder.h"
 #include "chrome/browser/ui/browser_window.h"
 #include "chrome/browser/ui/host_desktop.h"
 
 // How much horizontal and vertical offset there is between newly
 // opened windows.
 const int WindowSizer::kWindowTilePixels = 22;
 
 // static
 gfx::Point WindowSizer::GetDefaultPopupOrigin(const gfx::Size& size,
                                               chrome::HostDesktopType type) {
   NSRect work_area = [[NSScreen mainScreen] visibleFrame];
-  NSRect main_area = [[[NSScreen screens] objectAtIndex:0] frame];
+  NSRect main_area = [[[NSScreen screens] firstObject] frame];
   NSPoint corner = NSMakePoint(NSMinX(work_area), NSMaxY(work_area));
 
   if (Browser* browser = chrome::FindLastActiveWithHostDesktopType(type)) {
     NSWindow* window = browser->window()->GetNativeWindow();
     NSRect window_frame = [window frame];
 
     // Limit to not overflow the work area right and bottom edges.
     NSPoint limit = NSMakePoint(
         std::min(NSMinX(window_frame) + kWindowTilePixels,
                  NSMaxX(work_area) - size.width()),
         std::max(NSMaxY(window_frame) - kWindowTilePixels,
                  NSMinY(work_area) + size.height()));
 
     // Adjust corner to now overflow the work area left and top edges, so
     // that if a popup does not fit the title-bar is remains visible.
     corner = NSMakePoint(std::max(corner.x, limit.x),
                          std::min(corner.y, limit.y));
   }
 
   return gfx::Point(corner.x, NSHeight(main_area) - corner.y);
 }