[sk] Update asset download references to use SK

Change-Id: If3e838f22f1b99fd7a3b1c6bca0affd39f5573b0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/449843
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
4 files changed