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.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4063

Change-Id: I956949506200b766a2f7efb18e0486f3a2f93a1c
Reviewed-on: https://skia-review.googlesource.com/4063
Commit-Queue: Leon Scroggins <scroggo@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
3 files changed