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This implements slice header parsing and the rest of the overall decode
flow. It does not handle POCs, DPB, ref pic lists, etc. yet, those will
be added next.

BUG=chromium:1141237,b:153111783
TEST=media_unittests

Change-Id: Ib0173316ff8b25db9222d791e9dc8660ec282dad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518132
Commit-Queue: Jeffrey Kardatzke <jkardatzke@google.com>
Reviewed-by: Sergey Volk <servolk@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824690}