Default CORS mode based on as type for link preloads

[pmeenan]

This primarily affects fonts that are not same-origin where they default to loading as CORS anonymous when loaded by CSS and need the crossorigin attribute to be explicitly set when preloading. If it isn't specified then the preload will not match and the resource will be loaded again.

Since the preload knows the destination type, the preload CORS mode default logic should mirror the CORS logic for the given resource type.

This will mostly affect fonts but it would be good to have all resource preload types defer to the CORS processing of their underlying resource type.

[annevk]

Someone should study blame and figure out why this wasn't done when we added the attribute. I vaguely recall this has been discussed before.