Currently set cm->frame_refs_short_signaling as 0 by default. It will
write the one extra bit to the frame header, and leave all the encoder
setup the same as the baseline. It is in the TODO list for the encoder
side implementation for cm->frame_refs_short_signaling == 1.

BUG=aomedia:1392

Change-Id: Iabcc7618c9a52b7bb975b69e2d134f49a969944b