Should clear-site-data ever clear bfcache? /cc @fergald

Based on name/intent, I think executionContexts and cache might be good reasons to clear bfcache. (And arguably bfcache falls into the unspecified step 4 of cache.) But, I guess cache might be going away, er #68? And, per #59, nobody implements executionContexts?

We've run into some scenarios where people want to clear bfcache on logout. (I think @fergald can elaborate.) We thought using clear-site-data would be a good tool for this, but now I'm kind of confused.