Tested in the low latency setting. Comparing the three options:

(1). Cut off temporal mv altogether.
(2). Use collocated mv referncing.
(3). Use mfmv referencing.

The current baseline is using (1).

The results are:
(2) improves the compression performance over (1) by -0.59% for
lowres and -0.65% for midres.

(3) improves over (1) by -1.05% for lowres and -1.43% for midres

BUG=aomedia:1403

Change-Id: Ie683110ecbe16131f8aea123bca490f8897ebdd8