vp9: Add bias to last frame in choose_partitioning.
This change is only for real-time mode if short_circuit_low_temp_var is on. Add bias to last frame in choosing ref frame for partitioning, when y_sad and y_sad_g are close. It speeds up real-time encoding by 0.5% on some clips with less than 0.1% overall PSNR drop on rtc test set. Change-Id: I2a2110fe36455f3d8f0fc404aef2228f512e8df8