vp9-svc: Add frame dropper control to sample encoder.
vp9-svc: Add frame dropper control to sample encoder.

Disabled as default as enc_cfg.rc_dropframe_thresh is
set to 0 as default.

Change-Id: Ia888aa16b1a86a716ec33ea041e8b16b19bf93be

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