Support multiple media tracks in MSE / ChunkDemuxer

media/test/data/README

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 bear-320x240.webm - WebM encode of bear.1280x720.mp4 resized to 320x240.
 bear-320x240-video-only.webm - The video track of bear-320x240.webm.
 bear-320x240-audio-only.webm - The audio track of bear-320x240.webm.
 bear-vp9.webm - VP9 video only WebM file.
 bear-vp9-opus.webm - VP9 Video with Opus Audio.
 bear-vp8-webvtt.webm - WebM VP8 video with WebVTT subtitle track.
@@ skipping 185 matching lines @@
   video_encode_accelerator_unittest.
 
 // VP9 parser test files:
 bear-vp9.ivf
   - Created using "avconv -i bear-vp9.webm -vcodec copy -an -f ivf bear-vp9.ivf".
 bear-vp9.ivf.context 
 test-25fps.vp9.context
   - Manually dumped from libvpx with bear-vp9.ivf and test-25fps.vp9. See
     vp9_parser_unittest.cc for description of their format.
 
+// WebM files for testing multiple tracks.
+green-a300hz.webm - WebM file containing 10 seconds of solid green video + 300Hz sine wave audio
+red-a500hz.webm - WebM file containing 10 seconds of solid red video + 500Hz sine wave audio
+  - Created with the following commands:
+    ffmpeg -f lavfi -i color=c=red:size=320x240 -t 10 -c:v libvpx red.webm
+    ffmpeg -f lavfi -i color=c=green:size=320x240 -t 10 -c:v libvpx green.webm
+    ffmpeg -f lavfi -i "sine=frequency=300:sample_rate=48000" -t 10 -c:v libvpx a300hz.webm
+    ffmpeg -f lavfi -i "sine=frequency=500:sample_rate=48000" -t 10 -c:v libvpx a500hz.webm
+    ffmpeg -i green.webm -i a300hz.webm -map 0 -map 1 green-a300hz.webm
+    ffmpeg -i red.webm -i a500hz.webm -map 0 -map 1 red-a500hz.webm
+
 // JPEG test files:
 pixel-1280x720.jpg - Single MJEPG encoded frame of 1280x720, captured on Chromebook Pixel. This image does not have Huffman table.
 peach_pi-1280x720.jpg - Single MJPEG encoded frame of 1280x720, captured on Samsung Chromebook 2(13"). This image has Huffman table.
 blank-1x1.jpg - 1x1 small picture to test special cases.
 
 // MP4 files with non-square pixels.
 media/test/data/bear-640x360-non_square_pixel-with_pasp.mp4
   Size in TKHD is (639.2x360) and size in STSD is (470x360). A PASP box is
   present with hSpacing=34 and vSpacing=25. Note that 470.0 * 34 / 25 = 639.2.
 
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 media/test/data/bbb-320x240-2video-2audio.mp4
   Generated using following commands
   // Download the source file with 1 video and 1 audio stream.
   wget http://distribution.bbb3d.renderfarming.net/video/mp4/bbb_sunflower_1080p_30fps_normal.mp4
   // Generate a scaled down to 320x240 video + 2 channel AAC LC audio from the source file.
   ffmpeg -i bbb_sunflower_1080p_30fps_normal.mp4 -c:v libx264 -crf 36 -vf  scale=320:240 -c:a libfdk_aac -ac 2 -t 24 bbb1.mp4
   // Generate a file with the original video scaled down to 320x240 and flipped upside down and sine wave instead of audio.
   ffmpeg -i bbb_sunflower_1080p_30fps_normal.mp4 -f lavfi -i "sine=frequency=500:sample_rate=48000" -map 0:v -map 1:a -c:v libx264 -crf 36 -vf scale=320:240,vflip -c:a libfdk_aac -ac 2 -t 24 bbb2.mp4
   // Combine the two files generated above into a single fragmented .mp4 file with 2 video and 2 audio tracks.
   ffmpeg -i bbb1.mp4 -i bbb2.mp4 -map 0:0 -map 0:1 -map 1:0 -map 1:1 -c:v copy -c:a copy -movflags frag_keyframe+omit_tfhd_offset+separate_moof bbb-320x240-2video-2audio.mp4