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>
diff --git a/DEPS b/DEPS
index 802105d..7256fd0 100644
--- a/DEPS
+++ b/DEPS
@@ -45,3 +45,10 @@
 }
 
 recursedeps = [ "common" ]
+
+hooks = []
+
+import os
+import sys
+if os.path.exists('bin/fetch-gn'):
+  hooks.append({ 'action': [sys.executable, 'bin/fetch-gn'] })