rtc-vp9: Fix to integer overflow in vp9-svc

Divide by 3 instead of multiple by 3, in comparison of
lrc->avg_frame_bandwidth vd lrc->last_avg_frame_bandwidth,
in two functions for reset rc.
Small loss in precision, so acceptable.

Similar change to:
https://chromium-review.googlesource.com/c/webm/libvpx/+/5698570

Bug: chromium:367892770
Change-Id: Ia9ef09a9f6beba930fedd496407cfa7057e39336
2 files changed