vp9: Add bias to last frame in choose_partitioning.
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

Topopiccione/libvpx,smarter/aom,GrokImageCompression/aom,mbebenita/aom,mbebenita/aom,webmproject/libvpx,Topopiccione/libvpx,webmproject/libvpx,mwgoldsmith/vpx,mwgoldsmith/libvpx,luctrudeau/aom,mwgoldsmith/vpx,smarter/aom,mbebenita/aom,Topopiccione/libvpx,mwgoldsmith/libvpx,webmproject/libvpx,mbebenita/aom,webmproject/libvpx,GrokImageCompression/aom,smarter/aom,smarter/aom,webmproject/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,mbebenita/aom,mwgoldsmith/libvpx,mwgoldsmith/vpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,luctrudeau/aom,Topopiccione/libvpx,luctrudeau/aom,mwgoldsmith/vpx,mwgoldsmith/vpx,GrokImageCompression/aom,GrokImageCompression/aom,mbebenita/aom,mwgoldsmith/libvpx,smarter/aom,luctrudeau/aom,GrokImageCompression/aom,smarter/aom,GrokImageCompression/aom,ShiftMediaProject/libvpx,mbebenita/aom,mbebenita/aom,luctrudeau/aom,Topopiccione/libvpx,luctrudeau/aom,Topopiccione/libvpx,mbebenita/aom,mwgoldsmith/vpx,ShiftMediaProject/libvpx