nixos/tests/chromium: Fix the tests for Google Chrome

The meta attribute "timeout" is only set for Chromium (might still be required
due to the long build duration). The Google Chrome tests were failing with:

    error: attribute 'timeout' missing

According to nixos/lib/testing/meta.nix "null values are filtered out by
`meta`" so `timeout = chromiumPkg.meta.timeout or null` might be fine as
well.

nixos/tests/chromium.nix

@@ -39,7 +39,9 @@ mapAttrs (channel: chromiumPkg: makeTest {
   name = "chromium-${channel}";
   meta = {
     maintainers = with maintainers; [ aszlig primeos ];
+  } // optionalAttrs (chromiumPkg.meta ? timeout) {
     # https://github.com/NixOS/hydra/issues/591#issuecomment-435125621
+    # Note: optionalAttrs is used since meta.timeout is not set for Google Chrome
     inherit (chromiumPkg.meta) timeout;
   };