Fix view embedding crash

Author: arbreng

flow/layers/child_scene_layer.cc

@@ -19,11 +19,9 @@ ChildSceneLayer::ChildSceneLayer(zx_koid_t layer_id,
 
 void ChildSceneLayer::Preroll(PrerollContext* context, const SkMatrix& matrix) {
   TRACE_EVENT0("flutter", "ChildSceneLayer::Preroll");
-  set_needs_system_composite(true);
-
-  CheckForChildLayerBelow(context);
 
   context->child_scene_layer_exists_below = true;
+  CheckForChildLayerBelow(context);
 
   // An alpha "hole punch" is required if the frame behind us is not opaque.
   if (!context->is_opaque) {