Add fetch-gn to canvaskit and cmake compile scripts

If fetch-gn is omitted, the scripts fail in a clean checkout.

Change-Id: Idc944f7e8c5cd2ca998ab685b942df280e17dbab
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/197207
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Auto-Submit: Ben Wagner <benjaminwagner@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
2 files changed