Fix VideoSendStreamImpl::OnPacketFeedbackError: operator precedence.

in:
if (auto it = some_set.find(some_value) != some_set.end()) {}

"it" is bool, not an iterator.

Bug: webrtc:9652
Change-Id: Icfaab685f4e2f4456e24d5a14b11309dddfdc0fe
Reviewed-on: https://webrtc-review.googlesource.com/84420
Commit-Queue: Benoit Lize <lizeb@chromium.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24389}