Fix Metal builds.

shell/platform/darwin/ios/ios_surface_metal.mm

@@ -102,10 +102,7 @@
     return true;
   }
 
-  bool submitted = platform_views_controller->SubmitFrame(std::move(context),  // gr context
-                                                          {},  // onscreen gl context
-                                                          {}   // offscreen gl context
-  );
+  bool submitted = platform_views_controller->SubmitFrame(context, nullptr);
   [CATransaction commit];
   return submitted;
 }