Print preview: Use an ID instead of memory pointer string in WebUI.

BUG=144051

Review URL: https://chromiumcodereview.appspot.com/10870003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@153342 0039d316-1c4b-4281-b951-d872f2087c98
PrintPreviewHandler::~PrintPreviewHandler() {
  if (select_file_dialog_.get())
    select_file_dialog_->ListenerDestroyed();
}

PrintPreviewHandler::~PrintPreviewHandler() {
  if (select_file_dialog_.get())
    select_file_dialog_->ListenerDestroyed();
}
