webrtc: fix video fecBytesReceived stats

which were accidentally written to fecPacketsDiscarded

BUG=webrtc:42225568

Change-Id: Idc94e4490e77026a2de91f30346e2c283baf4231
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5609002


Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312624}