GitLab

Extract the useful interface from the V4L2ImageProcessor into a generic
interface that can be used for other kinds of processors and codecs.
Update the V4L2 encoder and decoder to use this more generic interface.

Also convert one instance of Callback into OnceCallback in the image
processor.

BUG=850375
BUG=b:73752373
TEST=Checked both VDA and VEA unittests on Hana.

Change-Id: Ia40a3989717f3cb03711b13a2ec574e018d2c614
Reviewed-on: https://chromium-review.googlesource.com/1084364Reviewed-by: Kuang-che Wu <kcwu@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569165}