vp9: Reduce overshoot factor for setting longer gf interval.
vp9: Reduce overshoot factor for setting longer gf interval.

Only affects 1 pass vbr mode.

Change-Id: Ifd7ff3771eabe322a371b35b159cace6baa3b070

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