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
5 files changed
tree: aad6e47c8254d7b722cf1aab50797f110ab25f71
  1. tools/