Add unit test for array narrowing conversions.

Change-Id: Ibadda2a10e19984c7a0efb66fd3187f280666445
Bug: skia:12248
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/436996
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
6 files changed