Update webpush test page tracking bug

The WebPush test pages need to be updated to use "aes128gcm" as well as "aesgcm".

This will require a bit of cleverness around javascript binary packing. It may also be worth dropping the key import function, since that's not chrome compatible anyway.

https://github.com/mozilla-services/WebPushDataTestPage/issues/16
https://github.com/jrconlin/Webpush_QA/issues/8