Skip frame bit-stream writing for show-existing frame
Skip frame bit-stream writing for show-existing frame

Make the bit-stream writer match the decoder behavior, when the
show existing frame feature is used.

Change-Id: Ibc8153f8668da0f9a2ed8af3b42dae91a5ac08c7

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