Update //tools/gn/docs/reference.md

With all of the latest GN features (like runtime deps).

R=brettw@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1169803005

Cr-Original-Commit-Position: refs/heads/master@{#333495}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 5316b8ce8569c25d02b018afff4b39aace1d8725