Add owners file to GN bootstrap. Add comment to the top of the script to clarify support and to suggest the type of thing that usually goes wrong. Review URL: https://codereview.chromium.org/1528863004 Cr-Original-Commit-Position: refs/heads/master@{#365290} Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: 484f322506039ab63ddfbcf3820932f561e5a0f9