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

ui/views/widget/native_widget_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 "ui/views/widget/native_widget_mac.h"
 
 #import <Cocoa/Cocoa.h>
 
 #import "base/mac/foundation_util.h"
 #import "base/mac/scoped_nsobject.h"
@@ skipping 1063 matching lines @@
 TEST_F(NativeWidgetMacTest, GetWorkAreaBoundsInScreen) {
   Widget widget;
   Widget::InitParams params = CreateParams(Widget::InitParams::TYPE_POPUP);
   params.ownership = Widget::InitParams::WIDGET_OWNS_NATIVE_WIDGET;
 
   // This is relative to the top-left of the primary screen, so unless the bot's
   // display is smaller than 400x300, the window will be wholly contained there.
   params.bounds = gfx::Rect(100, 100, 300, 200);
   widget.Init(params);
   widget.Show();
-  NSRect expected = [[[NSScreen screens] objectAtIndex:0] visibleFrame];
+  NSRect expected = [[[NSScreen screens] firstObject] visibleFrame];
   NSRect actual = gfx::ScreenRectToNSRect(widget.GetWorkAreaBoundsInScreen());
   EXPECT_FALSE(NSIsEmptyRect(actual));
   EXPECT_NSEQ(expected, actual);
 
   [widget.GetNativeWindow() close];
   actual = gfx::ScreenRectToNSRect(widget.GetWorkAreaBoundsInScreen());
   EXPECT_TRUE(NSIsEmptyRect(actual));
 }
 
 }  // namespace test
 }  // namespace views
 
 @implementation TestStopAnimationWaiter
 - (void)setWindowStateForEnd {
   views::test::ScopedSwizzleWaiter::GetMethodAndMarkCalled()(self, _cmd);
 }
 @end
 
 @implementation NativeWidgetMacTestWindow
 
 @synthesize invalidateShadowCount = invalidateShadowCount_;
 
 - (void)invalidateShadow {
   ++invalidateShadowCount_;
   [super invalidateShadow];
 }
 
 @end