gn: Document gen's --check flag.

BUG=698539

Review-Url: https://codereview.chromium.org/2872453003
Cr-Original-Commit-Position: refs/heads/master@{#470042}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 25e109413ae504e8b7aaf942169fa70d28fde6ed
1 file changed
tree: 791e73c18ad66446bb895eea97f4a11252351aa5
  1. tools/