[w3c/ServiceWorker] Get Cache Size (#1059)

I wish we had a simple method to get how much cache is used from a specific cache.

- Using `blob.size()` on each response from the cache isn't what I want as it uses the uncompressed size. 
- Using `content-length` headers is not reliable either as it is not always present. Even so, I'd have to concatenate all response headers (with some extra characters) and content-length headers which won't match exactly what is in the cache.

See my simple proposal. 

```js
caches.open('my-cache')
.then(cache => cache.getSize())
.then(cacheSize => console.log('My cache is about ' + cacheSize + ' Bytes'));
```

@jakearchibald WDYT?

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