Bug 493701 - part 1: add nsTObserverArray::BackwardIterator; r=bzbarsky

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extra : rebase_source : cb875dcd70b76b819abb1d516db74e55a3babd54
Bug 493701 - part2: Use TObserverArray for DocLoader Listeners; r=bzbarsky
* * *
Bug 493701 - part 3: add and use a BackwardIterator::Remove method; r=bzbarsky

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extra : rebase_source : 87e4e490602c10d7c833179901d85b9e3901e727