Testing of 4k videos encoded with a fixed arbitrary chunking interval
uncovered a bug where by if a chunk ends 1 frame before a real scene cut,
the next chunk may be encoded with two consecutive key frames at the start
with the first being assigned 0 bits.
This fix insures that where there is a key frame group of length 1 it is
at least assigned 1 frames worth of bits not 0.
See also patch Change-Id: I692311a709ccdb6003e705103de9d05b59bf840a
which by virtue of allowing fast adaptation of Q made this bug more visible.
BUG=webm:1456
Change-Id: Ic9e016cb66d489b829412052273238975dc6f6ab
db8fa86a6c