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

ui/events/cocoa/events_mac_unittest.mm

 // Copyright 2014 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/events/event_utils.h"
 
 #import <Cocoa/Cocoa.h>
 
 #include "base/mac/scoped_cftyperef.h"
 #import "base/mac/scoped_objc_class_swizzler.h"
@@ skipping 91 matching lines @@
                                       2,
                                       delta_y,
                                       delta_x));
     // CGEvents are always in global display coordinates. These are like screen
     // coordinates, but flipped. But first the point needs to be converted out
     // of window coordinates (which also requires flipping).
     NSPoint window_point =
         NSPointFromCGPoint(Flip(window_location).ToCGPoint());
     NSPoint screen_point = [test_window() convertBaseToScreen:window_point];
     CGFloat primary_screen_height =
-        NSHeight([[[NSScreen screens] objectAtIndex:0] frame]);
+        NSHeight([[[NSScreen screens] firstObject] frame]);
     screen_point.y = primary_screen_height - screen_point.y;
     CGEventSetLocation(scroll, NSPointToCGPoint(screen_point));
     return [NSEvent eventWithCGEvent:scroll];
   }
 
  private:
   scoped_ptr<base::mac::ScopedObjCClassSwizzler> swizzler_;
 
   DISALLOW_COPY_AND_ASSIGN(EventsMacTest);
 };
@@ skipping 182 matching lines @@
   event = cocoa_test_event_utils::MouseEventWithType(NSMouseMoved, 0);
   EXPECT_EQ(ui::ET_MOUSE_MOVED, ui::EventTypeFromNative(event));
 
   event = cocoa_test_event_utils::EnterExitEventWithType(NSMouseEntered);
   EXPECT_EQ(ui::ET_MOUSE_ENTERED, ui::EventTypeFromNative(event));
   event = cocoa_test_event_utils::EnterExitEventWithType(NSMouseExited);
   EXPECT_EQ(ui::ET_MOUSE_EXITED, ui::EventTypeFromNative(event));
 }
 
 }  // namespace ui