Update GN readme. Review URL: https://codereview.chromium.org/1805253002 Cr-Original-Commit-Position: refs/heads/master@{#381481} Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: ea79f96b9cd4b36bc0df5e3676fe2de4d63e95e4