Tyler Waring Robert is correct. ClassBreaksDefinition is used for generating renderers with the GenerateRendererTask. Since you're looking at a manual classification, you don't need to generate them at all. Instead you manually add them directly to the ClassBreaksRenderer. There's no need to use ClassBreaksDefinition in this case.
When you do need to generate a renderer with another classification method, I would actually recommend using the smartMapping.createClassedColorRenderer() (or sizeRenderer()) methods instead of the GenerateRendererTask. It's pretty straightforward and accomplishes the same task with fewer modules (again, no need for ClassBreaksDefinition in this scenario either).
esri/renderers/smartMapping | API Reference | ArcGIS API for JavaScript 3.18
I hope that helps!