call fetch-gn from git-sync-deps

https://skia.org/?cl=6994

Change-Id: Icac009bdef49f38ae7b8f082ffda6408481a03cd
Reviewed-on: https://skia-review.googlesource.com/6994
Reviewed-by: Hal Canary <halcanary@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed