Bug 832140 - 1/N: always call newTxn with store names array.
Bug 832140 - 2/N: reorder newTxn arguments as IndexedDBHelper::newTxn does.
Bug 832140 - 3/N: accept a successCb and a failureCb in ensureDb().
Bug 832140 - 4.a/N: accept a successCb and a failureCb in newTxn().
Bug 832140 - 4.b/N: fix init() and updatePendingTransactionToError().
Bug 832140 - 4.c/N: fix saveRecord().
Bug 832140 - 4.d/N: fix getMessageRecordByTransactionId().
Bug 832140 - 4.e/N: fix getMessageRecordById().
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