Update GN reference page.

Automatically generated with `gn help --markdown all`.

This refresh will add documentation for the new "friends" feature.

Bug: None
Change-Id: Ie330a2e53be8e8f5b76cab2211aa82bad6b47138
Reviewed-on: https://chromium-review.googlesource.com/1020526
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#552349}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: cd02e3d2d629291fda867a4099b4120a6d49902b