This patch uses the neighboring ref counts to design the contexts
for the coding of the first reference frame of a reference pair for
the compound prediction. This aligns the context design with that
for the second reference frame of a reference pair for the
compound prediction.

The new designed contexts are much simpler than that in the baseline.
The number of contexts for each binary symbol is reduced from 5 to
3. Further, the logic for each context only depends on the collected
neighboring ref counts, which is straightforward to derive.

The default CDFs for the first reference frame coding have been
updated using aom_entropy_optimizer.

Experimental results demonstrate a small coding gain for Google test
sets of both lowres and midres, with 30 frames coded for the default
coding tool setup:

lowres: avg_psnr -0.077%; ovr_psnr -0.076%; ssim -0.106%
midres: avg_psnr -0.059%; ovr_psnr -0.066%; ssim -0.037%

BUG=aomedia:1356

Change-Id: I781abbe4616dc3f3a7213ec663946ff9844eb830