Normally, assigning to e.g document.body.onload will forward to
window.onload. However, in a detached DOM tree, there is no associated
window, so we have nowhere to forward to, making this a no-op.
The bulk of this change is making Document::window() return a nullable
pointer, as documents created by DOMParser or DOMImplementation do not
have an associated window object, and so must be able to return null
from here.
Adds the initial implementation for interfaces defined in the
ResizeObserver specification. These interfaces will be used to
construct and send observation events in the upcoming changes.
