Bug 898871 - Hold a strong ref on the pres shell while scrolling, check for damage afterwards.  r=roc
Bug 898871 - Add an assertion that the pres shell was not deleted by some script runner, which should not happen here since the caller must hold a strong ref before starting the scroll operation.  r=roc
Bug 898871 - Add more nsWeakFrame checks around scroll operations.  r=roc