vp8: Change default gf behavior for 1 pass cbr.

vp8: Change default gf behavior for 1 pass cbr.

In 1 pass CBR, with error_resilience off, allow for
special logic to change the default gf behaviour.
In this CL: boost is turned off and the gf period
is set to a multiple of cyclic refresh period.

Change only affect 1 pass CBR mode, i.e, when the flag
gf_update_onepass_cbr is set.

Including the previous change (3ec8e11: to allow cyclic refresh
for error_resilience off), comparing metrics on RTC set for
error_resilience off vs on: avgPSNR/SSIM up by ~6%.

Change-Id: Id5b3fb62a4f04de5a805bd1b418f2b349574e0bc