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Add a small tracker that will always return the same V4L2 index when
given the same VideoFrame. This ensures that video buffers are always
assigned to the same V4L2 buffer, avoiding unmappings/remappings in the
kernel.

BUG=b:159688625
TEST=video.DecodeAccel.h264 passes on Kukui.

Change-Id: I4c439109891b0c02b607f2b238d21a4cca3a9c67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2413945
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Fritz Koenig <frkoenig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813078}