Generate and assign media track ids in demuxers.

media/filters/chunk_demuxer_unittest.cc

Index: media/filters/chunk_demuxer_unittest.cc
diff --git a/media/filters/chunk_demuxer_unittest.cc b/media/filters/chunk_demuxer_unittest.cc
index df13d2b2fa314047c9842385b856caa1f0a0c246..56f2c7ca78312beb0dd66ce370fdf83f5e4dc411 100644
--- a/media/filters/chunk_demuxer_unittest.cc
+++ b/media/filters/chunk_demuxer_unittest.cc
@@ -1405,6 +1405,13 @@ class ChunkDemuxerTest : public ::testing::Test {
     DCHECK(tracks.get());
     DCHECK_GT(tracks->tracks().size(), 0u);
 
+    // Verify that track ids are unique.
+    std::set<MediaTrack::Id> track_ids;
+    for (const auto& track : tracks->tracks()) {
+      EXPECT_EQ(track_ids.end(), track_ids.find(track->id()));
+      track_ids.insert(track->id());
+    }
+
     InitSegmentReceivedMock(tracks);
   }