This experiment of frame_sign_bias is to remove the 7 bits in the
frame header that signal the sign bias value for the 7 reference
frames. Instead of explicit signaling, this experiment is to use
the frame_offset value to derive the frame sign bias. It achieves
-0.08~-0.1% coding gain on average. It has been adopted by AOM.

Change-Id: I543f2641d9a11cabfe822cd4daa8b88b938421d6