The way we've been creating DOM::Document has been pretty far from what
the spec tells us to do, and this is a first big step towards getting us
closer to spec.
The new Document::create_and_initialize() is called by FrameLoader after
loading a "text/html" resource.
We create the JS Realm and the Window object when creating the Document
(previously, we'd do it on first access to Document::interpreter().)
The realm execution context is owned by the Environment Settings Object.
Used by Google seemingly almost all around account sign in and
management. The modern sign in page has this near the beginning:
```html
<base href="https://accounts.google.com">
```
All of the XHRs performed by sign in are relative URLs to this
base URL. Previously we ignored this and did it relative to the
current URL, causing the XHRs to 404 and sign in to fall apart.
I presume they do this because you can access the sign in page
from multiple endpoints, such as `/ServiceLogin` and
`/o/oauth2/auth/identifier`
The environment settings object is effectively the context a piece of
script is running under, for example, it contains the origin,
responsible document, realm, global object and event loop for the
current context. This effectively replaces ScriptExecutionContext, but
it cannot be removed in this commit as EventTarget still depends on it.
https://html.spec.whatwg.org/multipage/webappapis.html#environment-settings-object
