chromium: Improve update script and update to latest versions.
Previously, we had a single hash of the whole version response from
omahaproxy.
Unfortunately the dev version is released quite frequently, so the hash
is of no use at all (we could rather directly fetch rather than
executing the script, because it will fetch all channels anyway).
This pull request adds two methods of caching:
* First of all, if a perticular version/channel is already in the
previous version of the sources.nix file, don't download it again.
* And the second method is to check if the current sha256 is already
downloaded and reads the corresponding sha256 from the lookup table.
So, this should really help to avoid flooding the download servers and
to not stress impatient users too much.
f34225c440