Based on: https://codereview.qt-project.org/c/qt/qtwebengine-chromium/+/427771
Gentoo Bug: https://bugs.gentoo.org/831487

--- a/src/3rdparty/chromium/media/filters/ffmpeg_demuxer.h	2021-10-16 23:46:55.000000000 +0300
+++ b/src/3rdparty/chromium/media/filters/ffmpeg_demuxer.h	2023-01-23 17:34:49.122859630 +0200
@@ -145,6 +145,8 @@
   base::TimeDelta start_time() const { return start_time_; }
   void set_start_time(base::TimeDelta time) { start_time_ = time; }
 
+  int64_t first_dts() const { return first_dts_; }
+
  private:
   friend class FFmpegDemuxerTest;
 
@@ -202,6 +204,7 @@
   bool fixup_chained_ogg_;
 
   int num_discarded_packet_warnings_;
+  int64_t first_dts_;
   int64_t last_packet_pos_;
   int64_t last_packet_dts_;
 
--- a/src/3rdparty/chromium/media/filters/ffmpeg_demuxer.cc	2023-01-23 17:33:57.366213805 +0200
+++ b/src/3rdparty/chromium/media/filters/ffmpeg_demuxer.cc	2023-01-23 17:43:09.283436983 +0200
@@ -57,6 +57,8 @@
 
 namespace {
 
+constexpr int64_t kRelativeTsBase = static_cast<int64_t>(0x7ffeffffffffffff);
+
 void SetAVStreamDiscard(AVStream* stream, AVDiscard discard) {
   DCHECK(stream);
   stream->discard = discard;
@@ -88,7 +90,7 @@
       frames * base::Time::kMicrosecondsPerSecond / sample_rate);
 }
 
-static base::TimeDelta ExtractStartTime(AVStream* stream) {
+static base::TimeDelta ExtractStartTime(AVStream* stream, int64_t first_dts) {
   // The default start time is zero.
   base::TimeDelta start_time;
 
@@ -98,12 +100,12 @@
 
   // Next try to use the first DTS value, for codecs where we know PTS == DTS
   // (excludes all H26x codecs). The start time must be returned in PTS.
-  if (av_stream_get_first_dts(stream) != kNoFFmpegTimestamp &&
+  if (first_dts != AV_NOPTS_VALUE &&
       stream->codecpar->codec_id != AV_CODEC_ID_HEVC &&
       stream->codecpar->codec_id != AV_CODEC_ID_H264 &&
       stream->codecpar->codec_id != AV_CODEC_ID_MPEG4) {
     const base::TimeDelta first_pts =
-        ConvertFromTimeBase(stream->time_base, av_stream_get_first_dts(stream));
+        ConvertFromTimeBase(stream->time_base, first_dts);
     if (first_pts < start_time)
       start_time = first_pts;
   }
@@ -287,6 +289,7 @@
       fixup_negative_timestamps_(false),
       fixup_chained_ogg_(false),
       num_discarded_packet_warnings_(0),
+      first_dts_(AV_NOPTS_VALUE),
       last_packet_pos_(AV_NOPTS_VALUE),
       last_packet_dts_(AV_NOPTS_VALUE) {
   DCHECK(demuxer_);
@@ -353,6 +356,11 @@
   int64_t packet_dts =
       packet->dts == AV_NOPTS_VALUE ? packet->pts : packet->dts;
 
+  if (first_dts_ == AV_NOPTS_VALUE && packet->dts != AV_NOPTS_VALUE &&
+      last_packet_dts_ != AV_NOPTS_VALUE) {
+    first_dts_ = packet->dts - (last_packet_dts_ + kRelativeTsBase);
+  }
+
   // Chained ogg files have non-monotonically increasing position and time stamp
   // values, which prevents us from using them to determine if a packet should
   // be dropped. Since chained ogg is only allowed on single track audio only
@@ -1430,7 +1438,7 @@
 
     max_duration = std::max(max_duration, streams_[i]->duration());
 
-    base::TimeDelta start_time = ExtractStartTime(stream);
+    base::TimeDelta start_time = ExtractStartTime(stream, streams_[i]->first_dts());
 
     // Note: This value is used for seeking, so we must take the true value and
     // not the one possibly clamped to zero below.
@@ -1587,6 +1595,8 @@
   for (const auto& stream : streams_) {
     if (!stream || stream->IsEnabled() != enabled)
       continue;
+    if (stream->first_dts() == AV_NOPTS_VALUE)
+      continue;
     if (!lowest_start_time_stream ||
         stream->start_time() < lowest_start_time_stream->start_time()) {
       lowest_start_time_stream = stream.get();
@@ -1604,6 +1614,10 @@
     if (stream && stream->type() == DemuxerStream::VIDEO &&
         stream->IsEnabled()) {
       video_stream = stream.get();
+
+      if (stream->first_dts() == AV_NOPTS_VALUE)
+        continue;
+
       if (video_stream->start_time() <= seek_time) {
         return video_stream;
       }