Merge branch 'chromium'.

This introduces Chromium 39 as the new stable version along with a bunch
of fixes.

Fixes #2799, particularily the PDF plugin, which now is open source and
thus no longer an issue.

Also fixes #3219 and merges #2906, so we no longer get a crash while
trying to bring up the print preview dialog.

Thanks to @edwtjo for the CUPS version bump.

 * chromium: Switch to use open-source PDF plugin.
 * cups: bump 1.5.4 -> 1.7.5
 * chromium: Allow env vars for passing plugin paths.
 * chromium: Update all channels to latest versions.
 * protobuf: Clean up and update to version 2.6.1.

pkgs/misc/cups/default.nix

@@ -1,7 +1,7 @@
 { stdenv, fetchurl, pkgconfig, zlib, libjpeg, libpng, libtiff, pam, openssl
 , dbus, libusb, acl }:
 
-let version = "1.5.4"; in
+let version = "1.7.5"; in
 
 stdenv.mkDerivation {
   name = "cups-${version}";
@@ -10,7 +10,7 @@ stdenv.mkDerivation {
 
   src = fetchurl {
     url = "https://www.cups.org/software/${version}/cups-${version}-source.tar.bz2";
-    sha256 = "1rfhlv9b37120d6shngvyrcp99vh4a3lwdkrfanv3sjqid7068w0";
+    sha256 = "00mx4rpiqw9cwx46bd3hd5lcgmcxy63zfnmkr02smanv8xl4rjqq";
   };
 
   buildInputs = [ pkgconfig zlib libjpeg libpng libtiff libusb ]