Move XUL persistence handling into it's own class and make it a separate
nsIDocumentObserver so it can also be used in non-XUL documents.
To avoid adding persistence to all non-XUL documents, a document must add
the "mozpersist" attribute to the root element if it wants enable the
feature.
Differential Revision: https://phabricator.services.mozilla.com/D6802
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extra : moz-landing-system : lando