Add support for cancellation of demuxer reads.

media/filters/ffmpeg_demuxer.cc

Index: media/filters/ffmpeg_demuxer.cc
diff --git a/media/filters/ffmpeg_demuxer.cc b/media/filters/ffmpeg_demuxer.cc
index b34d8bc3cb784c81bdae8ca48c4c1ee5d77076b5..1033bbc5a999bdfaeecec45cfffcd0134b175f1a 100644
--- a/media/filters/ffmpeg_demuxer.cc
+++ b/media/filters/ffmpeg_demuxer.cc
@@ -592,6 +592,11 @@ void FFmpegDemuxerStream::FlushBuffers() {
   last_packet_duration_ = kNoTimestamp;
 }
 
+void FFmpegDemuxerStream::Abort() {
+  if (!read_cb_.is_null())
+    base::ResetAndReturn(&read_cb_).Run(DemuxerStream::kAborted, nullptr);
+}
+
 void FFmpegDemuxerStream::Stop() {
   DCHECK(task_runner_->BelongsToCurrentThread());
   buffer_queue_.Clear();
@@ -881,6 +886,23 @@ void FFmpegDemuxer::Initialize(DemuxerHost* host,
                  status_cb));
 }
 
+void FFmpegDemuxer::AbortPendingReads() {
+  DCHECK(task_runner_->BelongsToCurrentThread());
+
+  // This should only be called after the demuxer has been initialized.
+  DCHECK(blocking_thread_.IsRunning());
+  DCHECK_GT(streams_.size(), 0u);
+
+  // Abort all outstanding reads by returning EOS buffers (to avoid any errors
+  // being triggered in the pipeline).
+  for (auto* stream : streams_) {
+    if (stream)
+      stream->Abort();
+  }
+
+  data_source_->Abort();
+}
+
 void FFmpegDemuxer::Stop() {
   DCHECK(task_runner_->BelongsToCurrentThread());