GN: Update documentation for updating the gn binaries (the rolling process).

The doc is stale and there is a script to do this now.

BUG=None
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/1383003003

Cr-Original-Commit-Position: refs/heads/master@{#352533}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: d7e30d88c7b12ac86e7be77e2d6c2fd0207c24f3
1 file changed
tree: f637001f21a393b515e3ea3e1dc704c88b14c52b
  1. tools/