ML_VAR_PARTITION: enable at speed 5

ML_VAR_PARTITION: enable at speed 5

When the ML_VAR_PARTITION experiment is turned on, replace
REFERENCE_PARTITION with ML_BASED_PARTITION at speed 5.

Coding gains(avg_psnr) compared to baseline:
ytlivehr  1.63%
ytlivelr  0.07%

Tested encoding speed with several clips from ytlivehr and ytlivelr
on linux desktop(rt, vbr, 4 threads). Encoder speed is on average
faster than baseline:
360p:   14% faster
720p:    7% faster
1080p: 1.5% faster

Change-Id: I39b00078176ff516f7306818f33ba2b1ea53dfa1