Add "entropy" experiment

This patch added two features to improve entropy coding efficiency
for coefficient tokens.

1. Choose 1 of 4 default probability tables based on q-index for
key-frames.
It is ported from nextgen branch:
https://chromium-review.googlesource.com/#/c/280586/

2. Do backward update after each superblock (64X64) row using
subframe token counts.

Coding gain: 0.1% on lowres; 0.42% on midres; 0.36% on hdres.
Much larger gain for key-frames: 2.6%, 2.3%, 1.7%.

Design doc: go/huisu-entropy

Change-Id: Ia3b6a615636be09247d70e4c520405637561532b