[New API] Stream are going to be transferable eventually

[jimmywarting]

...That means no more annoying postMessage back and forth. instead of passing a messageChannel port to the service worker we can send a stream directly to the sw.js

// then we can replace this line
iframe.contentWindow.postMessage({filename, size}, '*', [channel.port2])

// with something along this:
iframe.contentWindow.postMessage({filename, size}, '*', [readableStream]) 
// and simply just pass that further down to the respondWith (that will be a direct data binding/flow)

[frank-dspeed]

it should be a additional option as not all browsers are supporting service workers