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

content/browser/web_contents/web_contents_view_mac.mm

 // Copyright (c) 2012 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 <Carbon/Carbon.h>
 
 #import "content/browser/web_contents/web_contents_view_mac.h"
 
 #include <string>
 
@@ skipping 104 matching lines @@
 void WebContentsViewMac::GetContainerBounds(gfx::Rect* out) const {
   // Convert bounds to window coordinate space.
   NSRect bounds =
       [cocoa_view_.get() convertRect:[cocoa_view_.get() bounds] toView:nil];
 
   // Convert bounds to screen coordinate space.
   NSWindow* window = [cocoa_view_.get() window];
   bounds.origin = [window convertBaseToScreen:bounds.origin];
 
   // Flip y to account for screen flip.
-  NSScreen* screen = [[NSScreen screens] objectAtIndex:0];
+  NSScreen* screen = [[NSScreen screens] firstObject];
   bounds.origin.y = [screen frame].size.height - bounds.origin.y
       - bounds.size.height;
   *out = gfx::Rect(NSRectToCGRect(bounds));
 }
 
 void WebContentsViewMac::StartDragging(
     const DropData& drop_data,
     WebDragOperationsMask allowed_operations,
     const gfx::ImageSkia& image,
     const gfx::Vector2d& image_offset,
@@ skipping 509 matching lines @@
   if (window && webContents && !webContents->IsBeingDestroyed()) {
     if ([window occlusionState] & NSWindowOcclusionStateVisible) {
       webContents->WasUnOccluded();
     } else {
       webContents->WasOccluded();
     }
   }
 }
 
 @end