Document that OpacityLayer's children are nonempty (flutter/engine#8707)

See https://github.com/flutter/flutter/issues/31517

engine/src/flutter/flow/layers/opacity_layer.h

@@ -9,6 +9,10 @@
 
 namespace flutter {
 
+// Don't add an OpacityLayer with no children to the layer tree. Painting an
+// OpacityLayer is very costly due to the saveLayer call. If there's no child,
+// having the OpacityLayer or not has the same effect. In debug_unopt build, the
+// |EnsureSingleChild| will assert if there are no children.
 class OpacityLayer : public ContainerLayer {
  public:
   OpacityLayer();