This adds an internals.loadURL(url) that defers Page::load so it starts
outside the calling task and can land between session-history traversal-
queue steps — as a load requested by ConnectionFromClient::load_url in
the UI process can, but as a load started from script never does.
Use case: Some session-history races are reachable only when a load
request arrives from the UI process between event-loop pumps — while the
session history traversal queue is mid-drain. A load started from script
enters navigate() inside the calling task, and claims the ongoing
navigation up front — so it can never land in that window. And so,
without this function, we can’t write tests for those kinds of races.
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