Use [UIScreen mainScreen] instead of [[UIScreen screens] objectAtIndex:0].

ui/gfx/screen_ios.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.
 
 #include "ui/gfx/screen.h"
 
 #import <UIKit/UIKit.h>
 
 #include "base/logging.h"
 #include "ui/gfx/display.h"
@@ skipping 51 matching lines @@
 
   // Returns the display that most closely intersects the provided bounds.
   virtual gfx::Display GetDisplayMatching(
       const gfx::Rect& match_rect) const OVERRIDE {
     NOTIMPLEMENTED();
     return gfx::Display();
   }
 
   // Returns the primary display.
   virtual gfx::Display GetPrimaryDisplay() const OVERRIDE {
-    UIScreen* mainScreen = [[UIScreen screens] objectAtIndex:0];
+    UIScreen* mainScreen = [UIScreen mainScreen];
+    CHECK(mainScreen);
     gfx::Display display(0, gfx::Rect(mainScreen.bounds));
     display.set_device_scale_factor([mainScreen scale]);
     return display;
   }
 
   virtual void AddObserver(gfx::DisplayObserver* observer) OVERRIDE {
     // no display change on iOS.
   }
 
   virtual void RemoveObserver(gfx::DisplayObserver* observer) OVERRIDE {
     // no display change on iOS.
   }
 };
 
 }  // namespace
 
 namespace gfx {
 
 Screen* CreateNativeScreen() {
   return new ScreenIos;
 }
 
 }  // namespace gfx