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 db320ba00e0e5ea5a0afccbcc2b0e2beb0222b25..68f460b8833d9f424155562e7ede672ffc18fd70 100644
--- a/media/filters/chunk_demuxer_unittest.cc
+++ b/media/filters/chunk_demuxer_unittest.cc
@@ -1404,6 +1404,13 @@ class ChunkDemuxerTest : public ::testing::Test {
     DCHECK(tracks.get());
     DCHECK_GT(tracks->tracks().size(), 0u);
 
+    // Verify that track ids are unique.
+    std::set<MediaTrack::TrackId> 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);
   }