When building with --enable-daala_ec, calls to aom_write_tree() and
 aom_read_tree() will convert a aom_tree_index structure with associated
 aom_prob probabilities into a CDF on the fly for use with the
 od_ec_encode_cdf_q15().
The number of symbols in the CDF is capped at 16, and trees that contain
 more than 16 leaf nodes are handled by splitting the most likely, e.g.,
 highest probability symbols, first and coding multiple symbols if
 necessary.

ntt-short-1:

         MEDIUM (%) HIGH (%)
    PSNR 0.000227   0.000213
 PSNRHVS 0.000215   0.000205
    SSIM 0.000229   0.000209
FASTSSIM 0.000229   0.000214

subset1:

          RATE (%)  DSNR (dB)
    PSNR -0.00026   0.00002
 PSNRHVS -0.00026   0.00002
    SSIM -0.00026   0.00001
FASTSSIM -0.00026   0.00001

Change-Id: Icb1a8cb854fd81fdd88fbe4bc6761c7eb4757dfe