At this point, we presave a generated password with empty username. 

We should save with the captured on the form username if it doesn't override any credential. Otherwise, presave with empty username. It may cause override as well, but it is acceptable. 

Username clearing
https://cs.chromium.org/chromium/src/components/autofill/content/renderer/password_generation_agent.cc?rcl=761e67a03549d289401b4a98409d2d8391a66dbb&l=381

https://cs.chromium.org/chromium/src/components/password_manager/core/browser/form_saver_impl.cc?rcl=af4be98f805ccd698af7faaad1bd948c78dc3a14&l=115 there is a bug. The code doesn't consider the case when password was generated and it is not a new login. Please check/add tests for that.