Add cross-origin opener policy - #5334

This commit adds the notion of cross-origin opener policy (COOP). COOP allows websites to restrict which origins they share their browsing context group with. annevk wrote a first draft of the behavior of COOP here: https://gist.github.com/annevk/6f2dd8c79c77123f39797f6bdac43f3e. This commit takes that draft and merges it into the spec.

Closes: #4580

• At least two implementers are interested (and none opposed):
  • Chrome
  • Mozilla
• [Tests] Tests have already been added at:
  • https://github.com/web-platform-tests/wpt/tree/master/html/cross-origin-opener-policy
• Implementation bugs are filed:
  • Chrome: https://crbug.com/922191
  • Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1613066
  • Safari: …

/browsers.html ( diff )
/browsing-the-web.html ( diff )
/dom.html ( diff )
/history.html ( diff )
/iana.html ( diff )
/index.html ( diff )
/infrastructure.html ( diff )
/offline.html ( diff )
/origin.html ( diff )
/references.html ( diff )
/webappapis.html ( diff )