Support the use of 2x2 prediction and transform block size for
chroma components. This experiment builds on top of cb4x4. It
brings back the loss in performance gains due to 4x4 chroma
component constrain. The coding gains for lowres are up from 1.5%
to 2.5%.

Change-Id: I8e461e230c2d920f6df1af9d935ebe9dfa3820d9