Fix VPX_KF_DISABLED.
Fix VPX_KF_DISABLED.

VP9 encoder still inserts key frame periodically when VPX_KF_DISABLED is
set in non SVC for 1-pass CBR.
BUG=webm:1592

Change-Id: Ie99d7c5b95230d739e263a2d87879693c53f620e

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