The original motivation for the Iterator/RemovingIterator split was that
PLDHashTable Checker class would treat them differently. But that didn't end up
happening (see bug 1131308). So this patch merges them. This is a small code
size win now but it will become bigger when I add iterators to nsTHashTable and
nsBaseHashtable.
The only complication is that PLDHashTable::Iter() is now non-const, which is
a problem if you use it in a const method. So I added PLDHashTable::ConstIter()
which is used in just two places. It's a bit of a hack -- effectively a
const_cast -- but I don't think it's too bad.
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