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

ui/base/test/ui_controls_mac.mm

Index: ui/base/test/ui_controls_mac.mm
diff --git a/ui/base/test/ui_controls_mac.mm b/ui/base/test/ui_controls_mac.mm
index 002cb03e646f0dbce4ebdbccacab5e039151effb..15cdb7963b019d11df0e749c13239fb6a40faafe 100644
--- a/ui/base/test/ui_controls_mac.mm
+++ b/ui/base/test/ui_controls_mac.mm
@@ -235,7 +235,7 @@ bool SendMouseMove(long x, long y) {
 bool SendMouseMoveNotifyWhenDone(long x, long y, const base::Closure& task) {
   CHECK(g_ui_controls_enabled);
   CGFloat screenHeight =
-    [[[NSScreen screens] objectAtIndex:0] frame].size.height;
+    [[[NSScreen screens] firstObject] frame].size.height;
   g_mouse_location = NSMakePoint(x, screenHeight - y);  // flip!
 
   NSWindow* window = WindowAtCurrentMouseLocation();