Doing so causes QtWebEngine to load its own view-source: page even if we supply
custom data.
Instead we pass the original page's URL (to not regress #2948).
This partially reverts #3521 and reintroduces TabData.viewing_source.
However, on QtWebEngine we can still ":view-source --pygments" and then
":view-source" (with or without "--pygments") again, because the bit gets
cleaned in _on_load_started.
See #3654.
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