An AppCache feature policy

[yoavweiss]

The existence of AppCache support in the platform is hurting loading performance by not sending out requests until the document element is available, even if they are available earlier. It would be highly beneficial for pages that don't have AppCache to be able to opt-out of that de-optimization.

Thoughts?

/cc @cramforce @clelland @kinu @RByers @dtapuska

[RByers]

AppCache is something I think we're going to want to eventually deprecate, so (like sync-xhr) it makes sense to have a feature policy for that alone. So I'm certainly supportive.

[kinu]

Oh yes, this could really have a perf benefit and we will be eventually deprecating the feature, totally supportive!!