nsIResumableChannel Interface

 Prepare this channel for resuming. The request will not start until
 asyncOpen or open is called. Calling resumeAt after open or asyncOpen
 has been called has undefined behaviour.

 @param startPos the starting offset, in bytes, to use to download
 @param entityID information about the file, to match before obtaining
  the file. Pass an empty string to use anything.

 During OnStartRequest, this channel will have a status of
  NS_ERROR_NOT_RESUMABLE if the file cannot be resumed, eg because the
  server doesn't support this. This error may occur even if startPos
  is 0, so that the front end can warn the user.
 Similarly, the status of this channel during OnStartRequest may be
  NS_ERROR_ENTITY_CHANGED, which indicates that the entity has changed,
  as indicated by a changed entityID.
 In both of these cases, no OnDataAvailable will be called, and
  OnStopRequest will immediately follow with the same status code.

void resumeAt(in unsigned long long startPos, in ACString entityID)