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This CL removes the usage of a GpuMemoryBufferFactory from the
V4L2VideoEncodeAccelerator. This is because we don't actually need it to
allocate buffers and it simplifies things.

Note that we must start the |encoder_task_runner_| with base::MayBlock()
because the first encoder to make use of this path will open the render
node.

Bug: b:173167846
Test: video.EncodeAccel.h264_720p_nv12_dmabuf on elm
Change-Id: I01d5f63f0b358c0ee9bb448ded7ba24355a72f9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2568548
Commit-Queue: Andres Calderon Jaramillo <andrescj@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835114}