This change fixes a bug that can be reproduced with the following steps:
```js
const iframe = document.createElement("iframe");
document.body.appendChild(iframe);
iframe.contentWindow.location.href = ("http://localhost:8080/demo.html");
```
These steps are executed in the following order:
1. Create iframe and schedule session history traversal task that adds
session history entry for the iframe.
2. Generate navigation id for scheduled navigation to
`http://localhost:8080/demo.html`.
3. Execute the scheduled session history traversal task, which adds
session history entry for the iframe.
4. Ooops, navigation to `http://localhost:8080/demo.html` is aborted
because addings SHE for the iframe resets the navigation id.
This change fixes this by delaying all navigations until SHE for a
navigable is created.
MediaQueryList will now remember if a state change occurred when
evaluating its match state. This memory can then be used by the document
later on when it's updating all queries, to ensure that we don't forget
to fire at least one change event.
This also required plumbing the system visibility state to initial
about:blank documents, since otherwise they would be stuck in "hidden"
state indefinitely and never evaluate their media queries.
The use of this HashMap looks very spooky, but let's at least use
finalize when cleaning them up on destruction to make things slightly
less dangerous looking.
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
Now that the heap has no knowledge about a JavaScript realm and is
purely for managing the memory of the heap, it does not make sense
to name this function to say that it is a non-realm variant.
