ARF overlays now use the same rate correction factor as regular inter
frames, further testing would be needed to see if it makes sense to use
a completely separate rate correction factor for ARF overlays.

$ vpxenc --cpu-used=5  --fps=50/1 --target-bitrate=2000
         parkjoy.y4m -o out.webm
=> Before: 3356 kb/s
=> After: 2271 kb/s

Change-Id: I73e4defa615ba7a8a2bdb845864f4b1721cbbffe