[WIP][iOS] Avoid nextDrawable take much time.

shell/platform/darwin/ios/ios_surface_metal_impeller.mm

@@ -52,7 +52,7 @@
   // When there are platform views in the scene, the drawable needs to be presented in the same
   // transaction as the one created for platform views. When the drawable are being presented from
   // the raster thread, there is no such transaction.
-  layer.presentsWithTransaction = [[NSThread currentThread] isMainThread];
+  layer.presentsWithTransaction = YES;
 
   return layer;
 }

shell/platform/darwin/ios/ios_surface_metal_skia.mm

@@ -66,7 +66,7 @@
   // When there are platform views in the scene, the drawable needs to be presented in the same
   // transaction as the one created for platform views. When the drawable are being presented from
   // the raster thread, there is no such transaction.
-  layer.presentsWithTransaction = [[NSThread currentThread] isMainThread];
+  layer.presentsWithTransaction = YES;
 
   return layer;
 }