pbutils: Fix vp9 mime mapping when caps when no chroma-site.
It's possible to have 4:2:0 chroma format without chroma-site in caps, but it isn't validated by the current code. This patch fixes it.
cc: @ystreet
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It's possible to have 4:2:0 chroma format without chroma-site in caps, but it isn't validated by the current code. This patch fixes it.
cc: @ystreet