vp8: Allow higher resolution to get periodic keyframe.
vp8: Allow higher resolution to get periodic keyframe.

BUG=webm:1632

Change-Id: Ib05a010245e77f9d502c3e7b8f488fca280ea544

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