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::RequestIDBCursorPrefetch(
    int n,
    WebIDBCallbacks* callbacks_ptr,
    int32 idb_cursor_id,
    WebExceptionCode* ec) {
  scoped_ptr<WebIDBCallbacks> callbacks(callbacks_ptr);

  int32 response_id = pending_callbacks_.Add(callbacks.release());
  Send(new IndexedDBHostMsg_CursorPrefetch(idb_cursor_id, CurrentWorkerId(),
                                           response_id, n, ec));
  if (*ec)
    pending_callbacks_.Remove(response_id);
}

void IndexedDBDispatcher::RequestIDBCursorPrefetch(
    int n,
    WebIDBCallbacks* callbacks_ptr,
    int32 idb_cursor_id,
    WebExceptionCode* ec) {
  scoped_ptr<WebIDBCallbacks> callbacks(callbacks_ptr);

  int32 response_id = pending_callbacks_.Add(callbacks.release());
  Send(new IndexedDBHostMsg_CursorPrefetch(idb_cursor_id, CurrentWorkerId(),
                                           response_id, n, ec));
  if (*ec)
    pending_callbacks_.Remove(response_id);
}
