Separate rate_correction_factor for boosted GFs

When the golden frame is boosted, the rate correction factor is not
correlated well with other inter frames even in CBR mode. This commit
changes to use GF specific rate_correction_factor when gf_cbr_boost
is greater than 20%.

Change-Id: I6312c1564387bcacc11f4c5e8a9cfdc781b5c3ab
Merge "Separate rate_correction_factor for boosted GFs"