Right now, RFHM::GetSiteInstanceForNavigation uses the last committed NavigationEntry to determine whether the current RFH is in view-source mode. This is wrong because the current RFH is not always the RFH in which the last navigation committed. For example, if a tab crashed we swap in a new RFH without modifying the entry. We should track whether a RFH is in view-source mode directly on the RFH.