Don't signal updated bitrate allocation on encoder paused

If the paced queue gets too long becaused of e.g. encoder overshoot,
the encoder is paused by setting the target bitrate to 0. Don't signal
this 0-bitrate via RTCP TargetBitrate message as the overall target
bitrate is probably unchanged.

(cherry picked from commit f4ef2dd)

Bug: webrtc:9734, chromium:883434
Change-Id: I77f23b707a8d4494d0c89fa05005ac1482eace52
Reviewed-on: https://webrtc-review.googlesource.com/99507
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Original-Commit-Position: refs/heads/master@{#24679}
Reviewed-on: https://webrtc-review.googlesource.com/100102
Cr-Commit-Position: refs/branch-heads/70@{open-webrtc-toolkit#8}
Cr-Branched-From: f18b352-refs/heads/master@{#24472}

video/video_send_stream_impl.cc

@@ -432,7 +432,10 @@ void VideoSendStreamImpl::SignalEncoderTimedOut() {
 
 void VideoSendStreamImpl::OnBitrateAllocationUpdated(
     const VideoBitrateAllocation& allocation) {
-  rtp_video_sender_->OnBitrateAllocationUpdated(allocation);
+  if (encoder_target_rate_bps_ != 0) {
+    // Send bitrate allocation metadata only if encoder is not paused.
+    rtp_video_sender_->OnBitrateAllocationUpdated(allocation);
+  }
 }
 
 void VideoSendStreamImpl::SignalEncoderActive() {