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This adds handling of CENCv1 (full sample encryption) streams for H264
with the VAAPI accelerator. The decoder detects when this case occurs
and then offloads slice header parsing to the accelerator.

BUG=b:153111783,b:155508443
TEST=Unit tests pass, CENCv1 playback works

Change-Id: Ieb913935269163d09d24b88a8ea987a6e8ffce33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2552634
Commit-Queue: Jeffrey Kardatzke <jkardatzke@google.com>
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833458}