Run fetch-gn in PRESUBMIT if needed

Change-Id: I3f2f3713fc27207c26d02c29358cd582440e8e65
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/271037
Commit-Queue: Eric Boren <borenet@google.com>
Auto-Submit: Ben Wagner aka dogben <benjaminwagner@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
1 file changed