fix chinese ime candidate window position problem.
we need the nsAutoPlaceHolderBatch destroctor called before hitting
GetCaretCoordinates so the states in Frame system sync with content
therefore, we put the nsAutoPlaceHolderBatch into a inner block
{
git-svn-id: svn://10.0.0.236/trunk@116055 18797224-902f-48f8-a5cc-f745e15eee43