Update GN documentation. This mostly updates the language.md file. Review URL: https://codereview.chromium.org/1841013003 Cr-Original-Commit-Position: refs/heads/master@{#383876} Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: 42998ec24cfcc5ced964448e77bc43bbdeca35dd