Support code show_existing_frame in bit-stream header
Support code show_existing_frame in bit-stream header

Allow the bit-stream writer to support potential use of
show_existing_frame. At this point, cm->show_existing_frame is
always 0.

Change-Id: I64fed1d72db6d4902d56774854ce24fb7a082e0c

webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx