Bug 844869 - Make NS_ASSERTION fatal under GTest. r=dbaron

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extra : rebase_source : e2a6772d505349be23a801b25fa1ee712a36a88e
Bug 834672 - AppProtocolHandler.js should never throw r=jduell
Bug 683159:  Explicitly gc in test_bug451286 so that intermittent assertions aren't spread across all the following tests.