Use the Object.getOwnPropertyNames() fix for PhantomJS in Chrome tests

See linked description in GitHub issue.

It is a bit silly that we are running the Chrome extension tests
exclusively in PhantomJS rather than Chrome, but for the moment,
get the existing tests to run using the same getOwnPropertyNames()
shim that is used for the main client app tests.

h/static/scripts/karma.config.js

@@ -73,6 +73,8 @@ module.exports = function(config) {
         bundle
           .transform('coffeeify')
           .plugin('proxyquire-universal')
+          // fix for Proxyquire in PhantomJS 1.x.
+          // See https://github.com/bitwit/proxyquireify-phantom-menace
           .require('phantom-ownpropertynames/implement', {entry: true});
       }
     },