Run bin/fetch-gn as a gclient hook.

Seems like the bots don't run hooks.  That's okay, even ideal.
They'll keep getting GN via recipes.

DOCS_PREVIEW= https://skia.org/?cl=5725

Change-Id: I000bad3390dddaeb4548972f29c96b8b3288ea6c
Reviewed-on: https://skia-review.googlesource.com/5725
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Eric Boren <borenet@google.com>
2 files changed