vp9-svc: Fix to frame dropping when layer is skipped.
vp9-svc: Fix to frame dropping when layer is skipped.

Fix condition in frame dropper for SVC to handle case
where spatial layer is skipped encoded (due to 0 bitrate).

Change-Id: I24185178774d73e8bb1c406acc0292422dfbe174

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