misc-fix: add fwd update of kf intra probs

This commit merges the fix that adds forward update of key frame intra
mode probabilities.

Related tracking issue
https://bugs.chromium.org/p/webm/issues/detail?id=1040
point #5
Original libvpx commit:
1eb51a2010c95845d340ac7dafe311e4c1716d66

Change-Id: If209b300f956ed3280f0249f6122ddfafe87f2a7