Support for new chromedriver downloads?

[dmurdoch]

In this issue ropensci/wdman#34 I pointed out that wdman can't download recent versions of chromedriver, because the Chrome developers have changed the way they are distributed. I think the new details can be found here: https://googlechromelabs.github.io/chrome-for-testing/.

I started looking into a PR for wdman to support the new scheme, but I see it uses binman::predl_google_storage for most of the work, so now I'm not sure if there should be a binman change as well to support the new scheme.