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

ui/gfx/color_profile_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.
 
 #import <Cocoa/Cocoa.h>
 
 #include "base/mac/scoped_nsobject.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/gfx/color_profile.h"
 #include "ui/gfx/mac/coordinate_conversion.h"
@@ skipping 33 matching lines @@
 
   BOOL TestWindowOnScreen() {
     return NSIntersectsRect(PrimaryScreenFrame(), [test_window() frame]);
   }
 
   BOOL TestWindowContainedOnScreen() {
     return NSContainsRect(PrimaryScreenFrame(), [test_window() frame]);
   }
 
   NSRect PrimaryScreenFrame() {
-    return [[[NSScreen screens] objectAtIndex:0] frame];
+    return [[[NSScreen screens] firstObject] frame];
   }
 };
 
 bool TestColorProfileForBounds(const gfx::Rect& bounds) {
   std::vector<char> color_profile;
   return gfx::GetDisplayColorProfile(bounds, &color_profile);
 }
 
 TEST_F(ColorProfileTest, GetDisplayColorProfileForOnScreenBounds) {
   MoveTestWindowTo(gfx::Rect(10, 10, 100, 100));
@@ skipping 69 matching lines @@
   EXPECT_FALSE(TestWindowOnScreen());
   EXPECT_FALSE(TestColorProfileForWindow(test_window()));
 }
 
 TEST_F(ColorProfileTest, GetDisplayColorProfileForNullWindow) {
   EXPECT_FALSE(TestColorProfileForWindow(nullptr));
   EXPECT_FALSE(TestColorProfileForWindow(nil));
 }
 
 }  // namespace