Bug 244945

Implement the concept of "browsing context set" from the HTML specification:
- https://html.spec.whatwg.org/multipage/browsers.html#browsing-context-set

For example, COOP violation reporting relies on the size of this set to determine if a violation report should be sent or not:
- https://html.spec.whatwg.org/multipage/origin.html#browsing-context-group-switches-due-to-cross-origin-opener-policy:coop-violation-navigation-from

"""
If browsingContext's group's browsing context set's size is greater than 1, then:
    * Queue a violation report for browsing context group switch when navigating to a COOP response with responseCOOP, "enforce", responseURL, currentCOOPEnforcementResult's url, currentCOOPEnforcementResult's origin, responseOrigin, and referrer.
"""

Right now, we have code in PingLoader that relies on Page::nonUtilityPageCount() instead, which is not correct.