Re: [fetch] Initializing context/content specific fetch defaults (#43)

Sure, let's make it concrete. The developer provides the destination context via "as" attribute, e.g..

```js
fetch('/gallery/photo.jpg', {as: 'image'}).then(...)
```
or
```html
<link rel=preload href=/gallery/photo.jpg as=image>
```

The user agent needs to...

1. Enforce CSP based on specified context? /cc @mikewest
2. Set appropriate HTTP request headers based on context - e.g. Accept, etc.
3. Set appropriate request priority: for HTTP/1.x this is for internal prioritization; for HTTP/2 this prioritization information is communicated to the server.

I know that in Blink we advertise different headers, and assign different priorities, based on type of resource being fetched. AFAIK, same applies to FF, WebKit, and IE. 

@pmeenan @mcmanus @toddreifsteck any thoughts or comments on this one?

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