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The test originally validates the entire bitstream. This CL
enables the test to validate each temporal layer stream. For
instance, when --num_temporal_layers=3, the test validates three
streams, TL0, TL0+TL1 and TL0+TL1+TL2.
BitstreamFileWriter and VideoFrameFileWriter ignore bitstreams
and frames in upper layers than to be processed by them.
VideoFrameFileWriter also writes a file with a different prefix
from other VideoFrameFileWriter, so that multiple instances don't
write to the same file names.

Bug: b:174318867
Test: video_encode_accelerator_tests --codec=vp9 --num_temporal_layers=3 w/o output_bitstream --output_images=all
Change-Id: Idf2a1e4d2a50e11ca359435ec4495a43b61139f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2562801Reviewed-by: David Staessens <dstaessens@chromium.org>
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833103}