[ServiceWorker] Save initial files into cache storage instead of browser's cache (#820)

At this time, there is no way to save intial files which are fetched at the very first HTTP request into CacheStorage I'm thinking about having a header that handles this issue
```
Cache-Control : location=cache-storage
```
Then developers can easily retrieve those files saved in cache storage on client side
```
CacheStrorageopen('main')then(cache => {
    cachematch('/path/to/initFile')then(response => {
        consolelog('File exists in cache storage');
    });
});

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/820