mChildSheetCount = mSheet->StyleSheetCount();
CSSParserImpl::SetStyleSheet(nsICSSStyleSheet* aSheet){  NS_PRECONDITION(nsnull != aSheet, "null ptr");  if (nsnull == aSheet) {    return NS_ERROR_NULL_POINTER;  }  if (aSheet != mSheet) {    // Switch to using the new sheet    NS_IF_RELEASE(mSheet);    mSheet = aSheet;    NS_ADDREF(mSheet);  }  return NS_OK;}