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
1 file changed
tree: 503d5c162a3b55de889c150c4b0cac3f00e0747b
  1. tools/