Extend Fetch API for labeled objects

[NAndreasson]

We need to extend the Fetch API to deal with receiving labeled objects, and also possibly sending. This would be similar to the XHR extension (http://w3c.github.io/webappsec-cowl/#extension-xhr).

[annevk]

The simplest way to do this would be to monkey patch Fetch for now https://annevankesteren.nl/2014/02/monkey-patch and later uplift it once it's clear two implementers are willing to implement this functionality and ship it.

You would need to make at least these changes:

• https://fetch.spec.whatwg.org/#dom-response-body needs to return null if the MIME type indicates a labeled object.
• https://fetch.spec.whatwg.org/#concept-body-consume-body needs to throw if the MIME type indicates a labeled object and type is not LabeledObject. (And you'll need to add a labeledObject() or some such method.)

[deian]

Awesome, thanks for the feedback @annevk!