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

ui/gfx/mac/coordinate_conversion_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.
 
 #import "ui/gfx/mac/coordinate_conversion.h"
 
 #import <Cocoa/Cocoa.h>
 
 #import "base/mac/scoped_objc_class_swizzler.h"
 #include "base/macros.h"
@@ skipping 30 matching lines @@
 
  private:
   scoped_ptr<base::mac::ScopedObjCClassSwizzler> swizzle_frame_;
 
   DISALLOW_COPY_AND_ASSIGN(MacCoordinateConversionTest);
 };
 
 void MacCoordinateConversionTest::SetUp() {
   // Before swizzling, do a sanity check that the primary screen's origin is
   // (0, 0). This should always be true.
-  NSRect primary_screen_frame = [[[NSScreen screens] objectAtIndex:0] frame];
+  NSRect primary_screen_frame = [[[NSScreen screens] firstObject] frame];
   EXPECT_EQ(0, primary_screen_frame.origin.x);
   EXPECT_EQ(0, primary_screen_frame.origin.y);
 
   swizzle_frame_.reset(new base::mac::ScopedObjCClassSwizzler(
       [NSScreen class],
       [MacCoordinateConversionTestScreenDonor class],
       @selector(frame)));
 
-  primary_screen_frame = [[[NSScreen screens] objectAtIndex:0] frame];
+  primary_screen_frame = [[[NSScreen screens] firstObject] frame];
   EXPECT_EQ(kTestWidth, primary_screen_frame.size.width);
   EXPECT_EQ(kTestHeight, primary_screen_frame.size.height);
 }
 
 void MacCoordinateConversionTest::TearDown() {
   swizzle_frame_.reset();
 }
 
 }  // namespace
 
@@ skipping 57 matching lines @@
   EXPECT_EQ(gfx_point, ScreenPointFromNSPoint(ns_point));
 
   // Point in a screen below and to the left primary screen.
   gfx_point = Point(-40, 220);
   ns_point = ScreenPointToNSPoint(gfx_point);
   EXPECT_NSEQ(NSMakePoint(-40, -20), ns_point);
   EXPECT_EQ(gfx_point, ScreenPointFromNSPoint(ns_point));
 }
 
 }  // namespace gfx