Bug 1632489 - Only adjust valid capture times when generating padding; r=bwc

The current code will only set the TranmissionOffset extension if
capture_time_ms is > 0, but when adjusting timestamps for rtx packets, it is
adjusted  without first checking to see if it is valid, which will cause invalid
values of capture_time_ms to be written to TranmissionOffset, leading to assertion
failures.

This bug is still present on tip of libwebrtc, so we'll also need to prepare a
patch for upstream.

Depends on D74842

Differential Revision: https://phabricator.services.mozilla.com/D75528

media/webrtc/trunk/webrtc/modules/rtp_rtcp/source/rtp_sender.cc

@@ -568,7 +568,9 @@ size_t RTPSender::SendPadData(size_t bytes,
         if (last_timestamp_time_ms_ > 0) {
           timestamp +=
               (now_ms - last_timestamp_time_ms_) * kTimestampTicksPerMs;
-          capture_time_ms += (now_ms - last_timestamp_time_ms_);
+          if (capture_time_ms > 0) {
+            capture_time_ms += (now_ms - last_timestamp_time_ms_);
+          }
         }
         if (!ssrc_rtx_) {
           RTC_LOG(LS_ERROR) << "RTX SSRC unset.";