Reland "Build SkRawCodec in GN"

Add BUILD.gn files for dng_sdk and piex and updated BUILD.gn to
build SkRawCodec.

We stopped testing raw images when we switched to GN, so this will
bring back our testing.

Leave SkRawCodec disabled on Windows, where we've had problems in the
past.

Originally landed in issue 4603. Reverted due to build errors.
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2463433002

Review-Url: https://codereview.chromium.org/2463433002
3 files changed