Update chrome status to beta m40

Hi there. I tried to do my best with my current SW skills and the given tests. I have a few remarks/questions :

1) I think the test about `postMessage` is not up-to-date. We should rename the resolved param `sw` to `registration` and call `registration.active.postMessage(...)`. I tried, it works but reading the spec I'm a bit lost to really confirm Chrome has the expected behaviour.

2) Do we need to pass the port in the message data? It's already available in the `event.ports`. What is the best practice regarding MessageChannels?

3) I updated the `fetchEvent.request` infos but maybe it's all there is.

4) `InstallEvent.replace()` seems to be present but I'm not sure it behaves properly.

5) I added the fetch polyfill by github.