GN: gpu support in tools

If you want to try this out, the DOCS_PREVIEW link is a quickstart.

Are the GrGLCreateNativeInteface_* meant to be part of Skia-the-library
or just used by testing tools?  I've got it written here as tool-only,
but we could easily move it to libskia if it's a thing we want to ship.

I'm honestly not sure I understand how this linked before without a definition
of GrGLCreateNativeInterface().  Or are there two, one real and one no-op
default with some sort of weak linking setup?  dlopen()?

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2201233003
DOCS_PREVIEW= https://skia.org/user/quick/gn?cl=2201233003

Review-Url: https://codereview.chromium.org/2201233003

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