Fixed metal error that came as the result of a revert.

shell/platform/darwin/ios/ios_surface_metal.mm

@@ -103,8 +103,7 @@
   }
 
   bool submitted = platform_views_controller->SubmitFrame(std::move(context),  // gr context
-                                                          {},  // onscreen gl context
-                                                          {}   // offscreen gl context
+                                                          {}  // gl context
   );
   [CATransaction commit];
   return submitted;