Move setting one permitted sandboxed navigator outside noopener branch

[MittelmanDaniel]

Fixes #12297.

Summary

Since #4330 made target="_blank" imply noopener=true, the "one permitted sandboxed navigator" is never set for sandboxed iframe popups because it lives inside the noopener=false ("Otherwise") branch of the rules for choosing a navigable.

This moves the step outside both branches so it runs regardless of noopener, matching Firefox's implementation and the existing placement of the "sandbox propagates to auxiliary browsing contexts" step which is already outside both branches.

Change

• Flattened the "Otherwise" block to a single step (just creating the traversable)
• Moved "If sandboxingFlagSet's sandboxed navigation browsing context flag is set, then set chosen's one permitted sandboxed navigator..." to its own step after both branches
• No other changes

Context

• The "one permitted sandboxed navigator" was originally fixed in Set "one permitted sandbox navigator" for all sandbox-created popups. #1774 (2016) to work for all sandbox-created popups
• Make target=_blank imply noopener; support opener #4330 (2019) made target="_blank" imply noopener, inadvertently putting all target="_blank" links on the code path that skips setting the permitted navigator
• Firefox handles this by setting the permitted navigator unconditionally before the noopener check (nsWindowWatcher.cpp)
• Found while fixing this bug in Ladybird

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