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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

Change-Id: I17704399c5724cbacc5611578e0dec22191e7a50
Reviewed-on: https://chromium-review.googlesource.com/c/1345701Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Commit-Queue: David Staessens <dstaessens@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610757}