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).
It is not guaranteed that inherited classes have the same address as the
base of the derived class. In that case a reinterpret cast leads to
undefined behavior. This occured on msvc ABI. See:
https://godbolt.org/z/jGeEW3c48
Co-authored-by: ayeteadoe <ayeteadoe@gmail.com>
