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

content/public/test/content_browser_test_utils_mac.mm

Index: content/public/test/content_browser_test_utils_mac.mm
diff --git a/content/public/test/content_browser_test_utils_mac.mm b/content/public/test/content_browser_test_utils_mac.mm
index 165a1776c9de0acf9928f1659a38431e69fac6b9..d2e879cfa3526bdd20a88b0a5625c60feaa9a15f 100644
--- a/content/public/test/content_browser_test_utils_mac.mm
+++ b/content/public/test/content_browser_test_utils_mac.mm
@@ -15,7 +15,7 @@ void SetWindowBounds(gfx::NativeWindow window, const gfx::Rect& bounds) {
   NSRect new_bounds = NSRectFromCGRect(bounds.ToCGRect());
   if ([[NSScreen screens] count] > 0) {
     new_bounds.origin.y =
-        [[[NSScreen screens] objectAtIndex:0] frame].size.height -
+        [[[NSScreen screens] firstObject] frame].size.height -
         new_bounds.origin.y - new_bounds.size.height;
   }