Comment 44 for bug 294712

I was assuming that we'd only veto content policy implementations we know about, hence the nsContentBlocker-specific solution.

Perhaps we could allow third-party content policies to opt-in to the override behavior by implementing this new interface, and have a helper similar to NS_CheckContentLoadPolicy that enumerates content policies, attempts to QI them, and calls IgnorePolicy. That's probably over engineered, though... I think a nsContentBlocker-specific solution would cover the majority case pretty well.