Protocol Buffers GN Build Support

This repository contains GN build system support for Protocol Buffers. It's in its own repository because it needs to be shared by Fuchsia and Cobalt.

This repo should be checked out such that:

• It is in //build/secondary/third_party/protobuf.
• Protobuf is in //third_party/protobuf.
• //.gn contains secondary_source = "//build/secondary/"

See the GN documentation on secondary_source.