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Creating a VideoFrame instance from a V4L2 buffer reference is a common
operation. The best place to do this is in the buffer reference itself,
since it can export the required DMABUFs itself, has a reference to its
own device, knows about its own layout, and can cache and reuse the
VideoFrame instance instead of creating a new one for each request.

This CL adds (and uses) such a method. Although it only works for MMAP
buffers, it will probably be extended in the future to other kinds of
buffers when they support queueing a VideoFrame directly.

Bug: 792790
Test: VDA unittest passed in regular and import mode on Hana.
Test: VEA unittest passed on Peach_pit.

Change-Id: If387d8f11f8a5c72d7fc26907f5c9bea4835d8b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1517503
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642386}