Bug 1007520 - Part 2: Avoid leaking Activities.callers in ActivitiesService.jsm. r=fabrice

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extra : rebase_source : f2a09304c40e743c5374ef2a52864b380b2f9a0a
Bug 1007520 - Part 3: Avoid leaking SpecialPowers to nsInProcessTabChildGlobal.mMessageManager. r=ted

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extra : rebase_source : f3f878a593c5c811698c2fd3cb978145e994386c