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

BUG=338626

Review URL: https://codereview.chromium.org/256813002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266686 0039d316-1c4b-4281-b951-d872f2087c98

ui/gfx/screen_ios.mm

@@ -69,7 +69,8 @@ class ScreenIos : public gfx::Screen {
 
   // 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;