Bug 1089332 - Add a getObservers API for accessing the history observers list. r=Mano

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extra : rebase_source : 990360d6c0e9b0fd5fc1dadc89f92d1edd52cd47
Bug 1090308 - Invalidate mDaysOfHistory when getObservers is invoked. r=Mano

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extra : rebase_source : 83b6c6aaaf1605b469a5023c01c909b9dbca46d4