Force the bit exactness in the first pass

Force the bit exactness in the first pass

Originally, for the purpose of keeping a fast first pass, the first-pass
stats between row_mt_mode = 0 and row_mt_mode = 1 are not bit exact, but
that difference is very small that doesn't cause a mismatch between the
final bitstreams. However, if the encoder changes, this minor difference
may cause a mismatch. Thus, this patch always forces the first pass to
be bit exact.

BUG=webm:1453

Change-Id: I2b67cf529dee81f660f9d9e7fe9a60ea3c7b12b8