[workbox-window] New messageSkipWaiting() method

[jeffposnick]

There's a subtle way that the "Offer a page reload for users" recipe can go wrong, in that the

wb.messageSW({type: 'SKIP_WAITING'});

could, under some circumstances, end up sending that message to the already-active service worker, not the service worker that's currently in the waiting state.

After discussing with @philipwalton, we're going to keep the messageSW() behavior the same (it will always call postMessage() on the service worker instance associated with the Workbox object), and instead come up with a new method—my proposed name is messageSkipWaiting()—that will be equivalent to using messageSW(sw, {type: 'SKIP_WAITING'}) with the sw parameter set to whatever the currently waiting service worker is.

We can then update the recipe to read

wb.messageSkipWaiting();

and it should behave as intended.