Adds the CfL experiment flag and computes a block-level DC_PRED that is
required by CfL in order to compute alpha_cb and alpha_cr.

The rate-distorsion impact of computing DC_PRED at the prediction block level
for chroma planes is rather small

Subset 1:
master_no_cdef@2017-04-18T20:37:05.712Z
  -> block_DCPRED_no_cdef@2017-04-18T20:38:07.381
  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
0.0712 |  0.0337 | -0.1692 |   0.0693 | 0.0814 |  0.0710 |    -0.0063
Note: CDEF was disabled because of problematic asserts.

Change-Id: I44d1cde8605b108366f4bd4cedbf5159dbbb5880