Remove 11 heavy includes from Document.h that were only needed for
pointer/reference types (already forward-declared in Forward.h), and
extract the nested ViewportClient interface to a standalone header.
This reduces Document.h's recompilation cascade from ~1228 files to
~717 files (42% reduction). Headers like BrowsingContext.h that were
previously transitively included see even larger improvements (from
~1228 down to ~73 dependents).
Previously we were using the document's window - this was both contrary
to spec and causing crashes when the document did not have a window (for
instance the `temp_document` in `HTMLParser::parse_html_fragment`.
This means we no longer crash when navigating between pages on
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