PSA: behavior change with Storage Access API implementation

[Chris Fredrickson]

FYI, I've just submitted https://crrev.com/c/3765690.

This CL changes the way that the COOKIES content setting is handled, so that Storage Access API permission grants do not affect the result of things like `contentSettings.cookies.get(frame_url, top_frame_url)`, but are still taken into account if needed when actually accessing cookies.