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This CL follows crrev.com/c/2393629 by batching up SubmitBuffer
calls in the H264 decode accelerator.

This is verified via chrome:tracing and codepen.io/miguelao/full/MMxGJw
that plays 4 1280x546 H.264 videos at the same time. Tracing is captured
for a few seconds, basically with this patch we have 2 SubmitBuffers()
calls adding up to ~0.461ms/frame versus ToT individual SubmitBuffer()
calls (4 of them) ~0.504ms in  total -- which gives an aggregate of
about ~40us improvement, or savings of about ~10%. Since decoding itself
takes about the 0.390ms/frame, savings are OKish, so let's take them.
This is all on my BSW device (reks),

Improvements are of course extremely small, the advantages of this CL
are in reducing lock/unlock churn and associated contention. This
benefit grows with the amount of decodes (e.g. Meet grid scenarios).

Bug: b/166646505
Change-Id: Id51bb6db39e4f7b9d67760f86b8f8963f20c2b2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2405586Reviewed-by: Jeffrey Kardatzke <jkardatzke@google.com>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806264}