[NORMATIVE] Set ref frame sign bias for S-frames
[NORMATIVE] Set ref frame sign bias for S-frames

There is no need to disable setting reference frame
sign bias for S-frames. In the typical use-case for
S-frames, all reference frames will be past, but
there is no need to change the behavior normatively.

With this change, S-frames simply become short cuts
for the following higher level flags:
error_resilient_mode = 1,
frame_size_override_flag = 1,
refresh_frame_flags = 0xff.

BUG=aomedia:1629

Change-Id: Ib1b1926836d2bbdd7a5ca485c8b476e918b5de7e

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom