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}