chromium/updater: Fix usage of insecure MD5 hashes

The title of https://github.com/NixOS/nix/issues/802

 says it all:

    Is it time to deprecate md5?

Of course it is - we shouldn't use MD5 for our beautiful updater, but
switch to SHA1 instead. It's the future!

       .    .          .       .           .
  .              .        .          .               .
    .    .           .           .        .-.   .         .     .
           -------     ______             | `\                .
.   ______   .            '   ``-..-.-,.-.`.  `----._______
           .  -------   .   . `.`.-=,'='-===| SHA1  \______`--._
 --------------      ---   .  ..-.-.-=-.-===|   ____________.--"
               ---------  . . .- .,==-'`-'-./  /__----'^  `^  `^  `^
   ____   -----     --  ____   ' . _____   \______[=>  =>  =>  =>
         .                   .       .                .
   .          .     .     .              .        .          .

Signed-off-by: aszlig <aszlig@nix.build>