RTCInboundRTPStreamStats's [fir/pli/nack]_count are collected for video.

Previously this was only collected for RTCOutboundRTPStreamStats video,
with no comment saying it was missing for Inbound. (nack_count should be
collected vor audio as well but this is currently not available - there
is already an existing comment about this in rtcstats_objects.h.)

BUG=chromium:657855, chromium:657854, chromium:627816

Review-Url: https://codereview.webrtc.org/2515293002
Cr-Original-Commit-Position: refs/heads/master@{#15185}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 820f578f2bb3585cc48876fa9e594a6a6b0dd174