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We now have support for NV12 overlays on all platforms where we have
atomic KMS. That means that we can always pick NV12 as the format to
decode to as it's the most efficient format for scanout and GPU
compositing.  Before we had NV12 overlays, decoding to RGB was a
better choice since that let us use an overlay, which was an overall
win.

Delete unit test PictureBufferFormat, since that's an unhelpful
change-detector test.

Bug: 813250
Change-Id: Ib6cb1cd2275693d9da04cd428081282cc1031be1
Reviewed-on: https://chromium-review.googlesource.com/950007
Commit-Queue: Kristian H. Kristensen <hoegsberg@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544226}