media/gpu/test: Add generic VideoFrameProcessor interface for testing.

To increase flexibility and to avoid code duplication, this CL adds a generic
VideoFrameProcessor interface. This interface will be used to write re-usable
test helpers that write video frames to files, calculate video frame checksums,
validate video frames,...

Currently the VideoFrameValidator is the only one using this interface but more
processors will be added soon. The video player API is slighty adapted to allow
multiple VideoFrameProcessors to be plugged in.

This CL also contains some minor cleanup.

TEST=ran new VDA tests on nocturne

BUG=879065

Change-Id: I376e0d5aefdfb5af966d1bde7facc5c9f7062340
Reviewed-on: https://chromium-review.googlesource.com/c/1404763
Commit-Queue: David Staessens <dstaessens@chromium.org>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623566}
11 files changed