The development team has been working on a feature that allows users more flexibility when it comes to the download process, including waiting for a Wi-Fi connection. Now that feature is live in Chrome Canary, so long as you enable a flag or two.
Toggling the "Enable download later" flag will make the new download prompt appear when on cellular, but you can also enable the "Show download later dialogue on WiFi" flag.
Once turned on, downloading a file will prompt you with options to start the download immediately, wait until connected to Wi-Fi, or at a certain day and time.
