Add Gr*Proxy classes

This isn't wired in anywhere yet.

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1937553002

Committed: https://skia.googlesource.com/skia/+/de5bf0cfeca908b81a28cc50065f7bc2da3d2fd1

Review-Url: https://codereview.chromium.org/1937553002
25 files changed