WebContent process keeps session history entries for pages we have
navigated away from. Before this change, those entries could prevent GC
objects (e.g. PolicyContainer and its CSP PolicyList) from being
collected, since the GC-allocated SHE/DocumentState held live GC::Ref
pointers into the heap.
By making both classes RefCounted and storing SerializedPolicyContainer
instead of a live PolicyContainer, history entries no longer keep alive
any GC objects. This eliminates the leak and is also a step toward
moving the session history entry tree to the UI process.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
