Declare intent to use sample-aes later in the stream.

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 107 matching lines @@
 bear-a_enc-a.webm - bear-320x240-audio-only.webm encrypted using key ID [1] and key [2].
 frame_size_change-av_enc-v.webm - third_party/WebKit/LayoutTests/media/resources/frame_size_change.webm encrypted using key ID [1] and key [2].
 
 bear-1280x720-hls.ts: produced using Apple's mediafilesegmenter tool with bear-1280x720.ts as input, with no encryption.
   mediafilesegmenter -t 10 -start-segments-with-iframe -f 'output_clear/' bear-1280x720.ts
 
 bear-1280x720-hls-sample-aes.ts: produced using Apple's mediafilesegmenter tool also with bear-1280x720.ts as input, but with SAMPLE_AES encryption.
   (Additional TS packets were constructed manually and prepended to convey the encryption metadata, notably key id and IV).
   mediafilesegmenter -S -P -k 'key_iv.bin' -t 10 -start-segments-with-iframe -f 'output/' bear-1280x720.ts
 
+bear-1280x720-hls-with-CAT.ts: same as bear-1280x720-hls.ts but with an extra TS packet prepended. This is the same as the first of the metadata packets in
+  bear-1280x720-hls-sample-aes.ts. Its presence indicates that SAMPLE_AES encryption may occur later in the stream, and causes the initial audio and video configs
+  to have an encryption_scheme (of AES-CBC). The actual data is unencrypted, which is indicated by the lack of key ID and IV. This ends up very similar to how
+  clear leader of an otherwise encrypted stream can occur in MP4.
+
 [1] 30313233343536373839303132333435
 [2] ebdd62f16814d27b68ef122afce4ae3c
 [3] KeyIds and Keys are created by left rotating key ID [1] and key [2] using
     std::rotate for every new crypto period. This is only for testing. The
     actual key rotation algorithm is often much more complicated.
 [4] http://www.webmproject.org/docs/webm-encryption/#46-subsample-encrypted-block-format
 
 // Container Tests (additional containers derived from bear.ogv)
 bear.ac3    -- created using "avconv -i bear.ogv -f ac3 -b 192k bear.ac3".
 bear.adts   -- created using "avconv -i bear.ogv -f adts -strict experimental bear.adts".
@@ skipping 176 matching lines @@
 
 // Multi-track WebM file
 multitrack-3video-2audio.webm
     //Generated using following commands:
     ffmpeg -f lavfi -i color=c=red:size=320x240 -t 5 -c:v libvpx red.webm
     ffmpeg -f lavfi -i color=c=green:size=320x240 -t 5 -c:v libvpx green.webm
     ffmpeg -f lavfi -i color=c=blue:size=160x120 -t 10 -c:v libvpx blue.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 5 -c:v libvpx a500hz.webm
     ffmpeg -i red.webm -i green.webm -i blue.webm -i a300hz.webm -i a500hz.webm -map 0 -map 1 -map 2 -map 3 -map 4  multitrack-3video-2audio.webm