chromium: add enable_hangout_services_extension=true to fix screensharing bug

as suggested in: https://code.google.com/p/chromium/issues/detail?id=416856#c53

pkgs/applications/networking/browsers/chromium/common.nix

@@ -174,6 +174,7 @@ let
     } // optionalAttrs proprietaryCodecs {
       # enable support for the H.264 codec
       proprietary_codecs = true;
+      enable_hangout_services_extension = true;
       ffmpeg_branding = "Chrome";
     } // optionalAttrs (stdenv.system == "x86_64-linux") {
       target_arch = "x64";