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

chrome/browser/ui/cocoa/omnibox/omnibox_popup_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 "chrome/browser/ui/cocoa/omnibox/omnibox_popup_view_mac.h"
 
 #include <cmath>
 
 #include "base/stl_util.h"
 #include "base/strings/sys_string_conversions.h"
@@ skipping 96 matching lines @@
 
   // Calculate the matrix size manually rather than using -sizeToCells
   // because actually resizing the matrix messed up the popup size
   // animation.
   DCHECK_EQ([matrix_ intercellSpacing].height, 0.0);
   PositionPopup(NSHeight([matrix_ frame]));
 }
 
 gfx::Rect OmniboxPopupViewMac::GetTargetBounds() {
   // Flip the coordinate system before returning.
-  NSScreen* screen = [[NSScreen screens] objectAtIndex:0];
+  NSScreen* screen = [[NSScreen screens] firstObject];
   NSRect monitor_frame = [screen frame];
   gfx::Rect bounds(NSRectToCGRect(target_popup_frame_));
   bounds.set_y(monitor_frame.size.height - bounds.y() - bounds.height());
   return bounds;
 }
 
 // This is only called by model in SetSelectedLine() after updating
 // everything.  Popup should already be visible.
 void OmniboxPopupViewMac::PaintUpdatesNow() {
   [matrix_ selectRowIndex:model_->selected_line()];
@@ skipping 169 matching lines @@
       IDR_OMNIBOX_STAR : AutocompleteMatch::TypeToIcon(match.type);
   return OmniboxViewMac::ImageForResource(resource_id);
 }
 
 void OmniboxPopupViewMac::OpenURLForRow(size_t row,
                                         WindowOpenDisposition disposition) {
   DCHECK_LT(row, GetResult().size());
   omnibox_view_->OpenMatch(GetResult().match_at(row), disposition, GURL(),
                            base::string16(), row);
 }