re-land "media/gpu/test: Cleanup and split test helpers build target."

> This change splits up the test helpers target in separate encode, decode and
> other helpers. Tests now only depend on what they actually need. This change
> doesn't introduce any new code, but some things are moved to avoid weird
> dependancies (such as encode helpers depending on frame mapper which depends on
> decode helpers).

> This change also moves code in the video_accelerator_unittest_helpers.h file
> from the media to the media/test namespace.
>
> TEST=ran video/jpeg encode/decode tests on nocturne
>
> CQ-DEPEND=CL:1345670
>
> BUG=879065,904337
>
> Change-Id: I17704399c5724cbacc5611578e0dec22191e7a50
> Reviewed-on: https://chromium-review.googlesource.com/c/1345701
> Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
> Commit-Queue: David Staessens <dstaessens@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#610757}

BUG=879065

Change-Id: I3c478745acdecc7bcbea089a079603c0c2472cdd
Reviewed-on: https://chromium-review.googlesource.com/c/1351193
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: David Staessens <dstaessens@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612518}