vp9: Modify ChangingDropFrameThresh unittest.
vp9: Modify ChangingDropFrameThresh unittest.

Add another (lower) bitrate to the test, to cover
frame drop behavior at low bitrate range.

Change-Id: Iaad003974159daf3d2d65ef3a6575a3e72e498d6

ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx