Check if there's a browsing context in window.open() steps

[zcorpan]

Fixes #11797.

• At least two implementers are interested (and none opposed):
  • Gecko
  • WebKit/Chromium: already implemented
• Tests are written and can be reviewed and commented upon at:
  • HTML: window.open() without a browsing context web-platform-tests/wpt#55475
• Implementation bugs are filed:
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1994707
• MDN issue is filed: I think https://developer.mozilla.org/en-US/docs/Web/API/Window/open#return_value doesn't need changes.
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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/nav-history-apis.html ( diff )

[annevk]

Is entry correct here (and below) or should it be this's relevant global object? (Which would require us to pass this.)

For instance, say a click handler in the parent stores iframe.contentWindow as childWindow, then calls iframe.remove(), and then calls childWindow.open(), this says that should work, right?

[zcorpan]

Need to write multiple-globals tests to check this.