Fix corrupt arf groups due to low "lag_in_frames"

Fix corrupt arf groups due to low "lag_in_frames"

Having a very small value for "lag_in_frames" can result in
corrupt arf groups including displayed frames that update
the arf buffer and fake overlay frames that are not in fact
overlays of real arfs but are nevertheless starved of bits.

Leaving lag_in_frames at the default of 25 for these 5 frame two
pass VBR tests should now give rise to a valid ARF coding pattern
as follows:-  K(ey), A(rf), N(ormal), N, N, O(verlay).

This change is part of a response to BUG=webm:1454 where broken
arf groups interacted badly with a change that corrects for large rate
misses. However, it may still in some cases increase encode time by
virtue of the fact that the unit test now codes a correct coding pattern
with "hidden" ARF frames.

Change-Id: Ifd0246a4c1d0be247247c754024d7a4ed5f66a6b