Add a //nogncheck anntation to GN include checker

The include checker will now skip includes annotated with "// nogncheck" which allows some edge cases to be worked around while still running the check pass on the rest of the includes.

This also updates the reference docs to the latest version.

Review URL: https://codereview.chromium.org/1217093007

Cr-Commit-Position: refs/heads/master@{#337499}
4 files changed