GitLab

Before Gitiles bug 82 was fixed, Gitiles did not render relative URLs
correctly on branches, so the GN docs had hard-coded absolute URLs.
That bug is now fixed, so this updates the docs.

R=brettw@chromium.org

Review-Url: https://codereview.chromium.org/2483673002
Cr-Commit-Position: refs/heads/master@{#439388}