How do we fetch resources specified in Link headers?

[mikewest]

According to https://w3c.github.io/preload/#processing, we should start "obtaining a resource" after processing a Link header. That algorithm relies on the existence of a Document, however, which we won't have until well after we process headers. Do we wait until the document is created before initiating requests? If not, how do we set the Fetch request's client, etc?

/cc @igrigorik @yoavweiss

[yoavweiss]

I think we should wait for Document here. This is how the implementations that I know are handling it.