As one of the first steps of removing XULDocument we can start loading all XUL files as XHTML first.

There are a few thing we'll need to update:

1. Anything that references XULDocument
2. All createElement calls will need to be converted to createXULElement

Now that the functionality we we wanted to preserve has been moved out of
XULDocument, we can start loading XUL files as XHTML. This will allow
the removal of XULDocument once we are confident that everything is working
correctly as XHTML.

test_bug819670_getter_throws.html:
Previously, the document context inspected in the console was the test
harness XULDocument, but now it will be an HTMLDocument. Update the
checks to make sure the inspected object has the same own properties as
the test documents own properties.

other tests:
Simply replace the expected document type.

Depends on D38694

For some extra testing, I pushed up a try run where for reftests I load one document as XHTML and one as XUL to ensure there were no regressions.

https://hg.mozilla.org/mozilla-central/rev/fd2b93ff0abe
https://hg.mozilla.org/mozilla-central/rev/4f0e19e164f1