Add DCHECK to ensure IndexedDBDispatcher doesn't get re-created.

This could happen if there are IDB objects that survive the call to
didStopWorkerRunLoop.

BUG=121734
TEST=


Review URL: http://codereview.chromium.org/9999035

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@131679 0039d316-1c4b-4281-b951-d872f2087c98
void IndexedDBDispatcher::CursorDestroyed(int32 cursor_id) {
  cursors_.erase(cursor_id);
}

void IndexedDBDispatcher::CursorDestroyed(int32 cursor_id) {
  cursors_.erase(cursor_id);
}
