vp9-svc: Add check in datarate unittests for frame-dropping.
vp9-svc: Add check in datarate unittests for frame-dropping.

Add verfication for constrained svc framedrop mode: check that
if a given spatial is dropped, all uppper layers must be dropped.

Change-Id: I9b4821b23c95d1d9d0c031a41af19984647ec5dc

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