Support chaining of render-blocking

[xiaochengh]

It would be very useful (or at least for web fonts, see use case) if we support chaining of render-blocking. The idea is that if:

• We have a blocking=render element A in document head (so it can successfully block rendering)
• A's subresource reponse contains a Link header that preloads resource B with blocking=render

Then we want to ensure that B can also successfully block rendering.

We currently can't ensure it. We currently don't process subresource link header (#7691). Another reason is that we allow adding render-blocking elements only if the body of the document hasn't been inserted yet. So there's a race condition that B can become render-blocking only if A's subresource response arrives before body is inserted.

To fix it:

• Need to resolve Proposal: add a boolean flag render-blocking to the request concept fetch#1433
• Add a "backdoor" to preload link header so that we can add it to the render-blocking element set even after body insertion, as long as the original subresource request is render-blocking (which guanrantees that the document is currently still render-blocked, so it's still safe to add new render-blocking elements).

[xiaochengh]

I'm not going to pursue this due to #7896.