In order to allow JS callers to use nsISimpleEnumerator instances with the JS
iteration protocol, we'll need to additional methods to every instance. Since
we currently have a large number of unrelated implementations, it would be
best if they could share the same implementation for the JS portion of the
protocol.

This patch adds a stub nsSimpleEnumerator base class, and updates all existing
implementations to inherit from it. A follow-up will add a new base interface
to this class, and implement the additional functionality required for JS
iteration.

Differential Revision: https://phabricator.services.mozilla.com/D3725

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extra : rebase_source : ad66d7b266856d5a750c772e4710679fab9434b1
extra : histedit_source : a83ebffbf2f0b191ba7de9007f73def6b9a955b8