GN support in the build system

GN is a third-party build tool used by chromium and some related projects that are vendored in mozilla-central. Rather than requiring GN to build or writing our own build definitions for these projects, we have support in the build system for translating GN configuration files into moz.build files. In most cases these moz.build files will be like any others in the tree (except that they shouldn’t be modified by hand), however those updating vendored code or building on platforms not supported by Mozilla automation may need to re-generate these files. This is a per-project process, described in dom/media/webrtc/third_party_build/gn-configs/README.md for webrtc. As of writing, it is very specific to webrtc, and likely doesn’t work as-is for other projects.