Use firstObject instead of objectAtIndex:0

atom/browser/native_window_mac.mm

@@ -626,7 +626,7 @@ NativeWindowMac::NativeWindowMac(
   options.Get(options::kWidth, &width);
   options.Get(options::kHeight, &height);
 
-  NSRect main_screen_rect = [[[NSScreen screens] objectAtIndex:0] frame];
+  NSRect main_screen_rect = [[[NSScreen screens] firstObject] frame];
   NSRect cocoa_bounds = NSMakeRect(
       round((NSWidth(main_screen_rect) - width) / 2) ,
       round((NSHeight(main_screen_rect) - height) / 2),
@@ -937,7 +937,7 @@ void NativeWindowMac::SetBounds(const gfx::Rect& bounds, bool animate) {
 
   NSRect cocoa_bounds = NSMakeRect(bounds.x(), 0, size.width(), size.height());
   // Flip coordinates based on the primary screen.
-  NSScreen* screen = [[NSScreen screens] objectAtIndex:0];
+  NSScreen* screen = [[NSScreen screens] firstObject];
   cocoa_bounds.origin.y =
       NSHeight([screen frame]) - size.height() - bounds.y();
 
@@ -947,7 +947,7 @@ void NativeWindowMac::SetBounds(const gfx::Rect& bounds, bool animate) {
 gfx::Rect NativeWindowMac::GetBounds() {
   NSRect frame = [window_ frame];
   gfx::Rect bounds(frame.origin.x, 0, NSWidth(frame), NSHeight(frame));
-  NSScreen* screen = [[NSScreen screens] objectAtIndex:0];
+  NSScreen* screen = [[NSScreen screens] firstObject];
   bounds.set_y(NSHeight([screen frame]) - NSMaxY(frame));
   return bounds;
 }