[css-view-transitions-2] Dealing with promises on old VT object

[mozilla-apprentice]

A resolution was made for csswg-drafts/#9888.

[css-view-transitions-2] Dealing with promises on old VT object

• RESOLVED: Keep the existing VT object for MPA transitions; the promises resolve when back from BFCache, ready rejects. acts like a skipped transition when document hides

Discussion.

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