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

ui/base/test/ui_controls_mac.mm

 // Copyright 2013 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 "ui/base/test/ui_controls.h"
 
 #import <Cocoa/Cocoa.h>
 #include <vector>
 
 #include "base/bind.h"
@@ skipping 217 matching lines @@
   return SendMouseMoveNotifyWhenDone(x, y, base::Closure());
 }
 
 // Input position is in screen coordinates.  However, NSMouseMoved
 // events require them window-relative, so we adjust.  We *DO* flip
 // the coordinate space, so input events can be the same for all
 // platforms.  E.g. (0,0) is upper-left.
 bool SendMouseMoveNotifyWhenDone(long x, long y, const base::Closure& task) {
   CHECK(g_ui_controls_enabled);
   CGFloat screenHeight =
-    [[[NSScreen screens] objectAtIndex:0] frame].size.height;
+    [[[NSScreen screens] firstObject] frame].size.height;
   g_mouse_location = NSMakePoint(x, screenHeight - y);  // flip!
 
   NSWindow* window = WindowAtCurrentMouseLocation();
 
   NSPoint pointInWindow = g_mouse_location;
   if (window)
     pointInWindow = [window convertScreenToBase:pointInWindow];
   NSTimeInterval timestamp = TimeIntervalSinceSystemStartup();
 
   NSEvent* event =
@@ skipping 85 matching lines @@
 void RunClosureAfterAllPendingUIEvents(const base::Closure& closure) {
   base::MessageLoop::current()->PostTask(
       FROM_HERE, base::Bind(&EventQueueWatcher, closure));
 }
 
 bool IsFullKeyboardAccessEnabled() {
   return [NSApp isFullKeyboardAccessEnabled];
 }
 
 }  // namespace ui_controls