media/test/data: Add av1 test data

Add two av1 bitstreams for av1_decoder_unittest.
av1-svc-L2T2.ivf has temporal and spatial layers. It is copied from test vectors
in libaom project [1]. av1-show_existing_frame.ivf has frames with
existing_show_frame=1. It is copied from Timothy's av1 samples [2].

[1] https://aomedia.googlesource.com/aom/+/master/test/test_vectors.cc
[2] https://people.xiph.org/~tterribe/av1/samples-all/

Bug: 1000988
Test: media_unittests on chromeos
Change-Id: I533fa9e7722182f9fa060ae73e7eff1f3fdb292c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918906Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716062}

media/test/data/README.md

@@ -162,6 +162,17 @@ ffmpeg -i bear-av1.mp4 -i bear-opus.mp4 -c copy -strict -2 \
 **Note**: "-strict -2" was required because the current ffmpeg version
 has support for OPUS in MP4 as experimental.
 
+#### av1-svc-L2T2.ivf
+AV1 data that has spatial and temporal layers.
+This is the same as av1-1-b8-22-svc-L2T2.ivf in
+[libaom test vectors]:https://aomedia.googlesource.com/aom/+/master/test/test_vectors.cc
+
+#### av1-show_existing_frame.ivf
+AV1 data that contains frames with `show_existing_frame=1`.
+This is the same as 00000592.ivf in
+https://people.xiph.org/~tterribe/av1/samples-all/
+
+
 ### Alpha Channel
 
 #### bear-vp8a.webm