Transport sequence number should be set also for retransmissions.

This is a reland of https://codereview.webrtc.org/1385563005 which was
reverted since the test was flaky. The reason was a race condition (in
the test) and that NACK wasn't properly set up.

BUG=

Review URL: https://codereview.webrtc.org/1406193002

Cr-Original-Commit-Position: refs/heads/master@{#10303}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 861c55e58311383b7f4f61af463ddea53eb3f30f
2 files changed