vp9: Add datarate test for 1 pass VBR mode.
vp9: Add datarate test for 1 pass VBR mode.

Existing tests are only for CBR mode.

Change-Id: Ie3b2cd46236457748e2650901d1a347a730f38af

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