Normalize the target step again at the end of applying a history
step, since iframe removal can leave the originally computed target
step unused before the asynchronous application finishes. Let the UI
history snapshot use the same used-step mapping when it serializes a
current item for the UI process.
Handle late child-frame navigation commits whose original nested
history entry disappeared before finalization. Removed iframes have no
live nested history list to update, and initial about:blank still needs
its first real navigation to replace the remaining initial child entry.
Add coverage for iframe pushState before nested history readiness and
for removing and recreating an iframe after an initial same-document
history update.
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