Replace 'preload' context with context calculated from the |as| attribute

[briansmith]

<link rel=preload> was changed to make the as attribute mandatory. This was done so that the browser can determine what context-sensitive parameters to supply to the fetch. We should take advantage of this by calculating a fetch context from the as attribute instead of having a preload fetch context. For example, ``should usestyle` as the fetch context. And, in particular, there should be no `preload` context. This will help ensure that all of the fetch's security checks, prioritization logic, caching header setting, etc. are consistent with what would happen in the non-preload case, so that a response from a preload will more exactly match the response that would be given from a non-preload fetch.

Note that a boolean "is this a preload" flag is still needed so that things like mixed content blocking can treat preloads specially.

Note that this would require a change to MIX, at least, because MIX should continue to block mixed-content preloads. (It already requires them to be blocked since 'preload' is not on the whitelist.) /cc @mikewest.

[igrigorik]

See: #64 (comment).

tl;dr: the plan is to replace "as" with "type". The context will (and needs to) remain as "preload".

[briansmith]

I'm not convinced it is a good idea to continue using the "preload" context but I do agree that #64 subsumes this issue. I will comment in #64 later.