Support chrome variants in fetcher

[Sytten]

Since I last wrote the fetcher, chrome added new distributions for us to use instead of the good old chromium.
We can use chrome-driver, chrome-headless-shell and chrome. They are all available on https://googlechromelabs.github.io/chrome-for-testing/ (bucket https://storage.googleapis.com/chrome-for-testing-public).
This drastically reduces the maintenance burden of tracking which chromium revision has an actual build and also adds support for linux aarch64 (fixes #238).

I will work on it this november, it should be fairly easy to add.

[Sytten]

Update after a couple hours, it looks like since puppeteer switched to chrome-for-testing the chromium builds are all over the place and finding revision that was available on all platforms is hard. They also shutdown the proxy that people were using to get the revisions (https://omahaproxy.appspot.com/all.json?channel=${channel}&os=linux)
Links for references:
-https://chromiumdash.appspot.com/commits
-https://github.com/puppeteer/puppeteer/blob/bc93722c244ded3c3c8d67fd40ad744f19e86ba7/tools/check_availability.js
-https://commondatastorage.googleapis.com/chromium-browser-snapshots/index.html?prefix=Mac/
-https://source.chromium.org/chromium/chromium/src