fix assertions when manipulating the MathML attribute style sheet, b=188103, r+sr=bzbarsky

  cssSheet->SetDefaultNameSpaceID(kNameSpaceID_MathML);

  nsCOMPtr<nsIStyleSheet> sheet(do_QueryInterface(cssSheet));

  // all done, no further activity from the net involved, so we better do this

  sheet->SetComplete();

  // insert the stylesheet into the styleset without notifying observers

  styleSet->AppendAgentStyleSheet(sheet);

  *aSheet = sheet;

      // that may come from reconstructing the frame tree. Our rules only need

      // a re-resolve of style data and a reflow, not a reconstruct-all...

      sheet->SetOwningDocument(nsnull);

      // We're about to manipulate the CSSOM, so we better do this

      sheet->SetComplete();

    }

    // check for duplicate, if a similar rule is already there, don't bother to add another one