Join/Leave request ordering details in explainer are inaccurate.

[MattMenke2]

The explainer says "Each frame must, however, run all pending joins and leaves for a single owner in the order in which they were made. Same-origin operations should be applied immediately." Each attempts to make sure that the last 20 cross-origin operations at least make it to the browser process (though there are potential races with closing the frame, since the pipe to send those messages is frame-scoped, but a different pipe from navigations).

Currently, there are separate cross-origin join and leave queues (with a new clear queue for #829), with no interaction between them.

We merge the queues into one and make the spec more explicit about there being single a queue for all operations, or have the explainer allow multiple queues. We could also try to modify the queuing mechanism to have separate queues but not allow joins/leaves from the same origin across queues to jump in front of each other, but I think that just gets too complicated.

We should also add mention of clearOriginJoinedInterestGroups in the queuing description. I'm not adding that to #829, because I think we want to resolve this issue before/while adding in mention of the new method.

Note that changing queuing behavior could theoretically be a breaking change for some consumers, though I hope nothing has ossified so much as to strongly depend on current behavior yet.