Fix low-latency renderer with unset playout delays

The low-latency renderer is activated by the RTP header extension
playout-delay if the min value is set to 0 and the max value is
set to something greater than 0.

According to the specification of the playout-delay header
extension it doesn't have to be set for every frame but only if
it is changed. The bug that this CL fixes occured if a playout
delay had been set previously but some frames without any specified
playout-delay were received. In this case max composition delay
would not be set and the low-latency renderer algorithm would be
disabled for the rest of the session.

Bug: chromium:1138888
Change-Id: I12d10715fd5ec29f6ee78296ddfe975d7edab8a9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/208581
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Johannes Kron <kron@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33330}

modules/video_coding/generic_decoder.h

@@ -35,7 +35,6 @@ struct VCMFrameInformation {
   void* userData;
   VideoRotation rotation;
   VideoContentType content_type;
-  PlayoutDelay playout_delay;
   EncodedImage::Timing timing;
   int64_t ntp_time_ms;
   RtpPacketInfos packet_infos;