Update reference doc page for GN. R=brettw@chromium.org Review URL: https://codereview.chromium.org/1327733003 Cr-Original-Commit-Position: refs/heads/master@{#347459} Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: 3473904f7c2ee133a84b6820074270996be9ec46