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For some platform specific format, e.g. V4L2_PIX_FMT_MT21C, it does not
have corresponding VideoPixelFormat mapping. As the format is only used
between video decoder accelerator (VDA) and image processor, we shall
add an optional field, fourcc, in PortConfig so that VDA can tell image
processor which input format it should be used.

BUG=985683, b:132589320
TEST=Run VDA_unittest and media_unittest on elm

Change-Id: I1362a6805b64bab03db254010e7b62b0cc4fd68a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1724269
Commit-Queue: Shuo-Peng Liao <deanliao@chromium.org>
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682596}