vp9-rtc: Reduce color artifact for speed 8
vp9-rtc: Reduce color artifact for speed 8

Push the reduced chroma check to speed > 8.

Change-Id: I92dd0aa9933bb5417b1dc5eef8f805ee51e04ac9

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