Add casting to fix warning.
Add casting to fix warning.

Frame bits can safely be stored int but group bits
(kf or arf) use 64bit.

Change-Id: I0800f2a28070f8749110a95721c116fc56987885

Topopiccione/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,Topopiccione/libvpx,mwgoldsmith/vpx,webmproject/libvpx,Topopiccione/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,mwgoldsmith/vpx,ShiftMediaProject/libvpx,Topopiccione/libvpx,mwgoldsmith/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,mwgoldsmith/vpx,mwgoldsmith/libvpx,webmproject/libvpx,Topopiccione/libvpx,Topopiccione/libvpx,webmproject/libvpx,mwgoldsmith/libvpx,webmproject/libvpx,mwgoldsmith/vpx,mwgoldsmith/vpx,ShiftMediaProject/libvpx,mwgoldsmith/vpx,mwgoldsmith/libvpx