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).
These are not associated with a javascript realm, so to avoid
confusion about which realm these need to be created in, make
all of these objects a GC::Cell, and deal with the fallout.
These form the basis of Content Security Policy. A policy is a
collection of directives that are parsed from either the
Content-Security-Policy(-Report-Only) HTTP header, or the `<meta>`
element.
The directives are what restrict the operations can be performed in the
current global execution context. For example, "frame-ancestors: none"
tells us to prevent the page from being loaded in an embedded context,
such as `<iframe>`.
You can see it a bit like OpenBSD's pledge() functionality, but for the
web platform: https://man.openbsd.org/pledge.2
