Update iOS test apps to require C++17.

Fixes these build breaks: http://screen/5kZxPUFhoSrwpZ8

Change-Id: Ib4c92c0c33c9b0fdd639092e901f475e6f6bc655
Bug: skia:12882
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/502307
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
2 files changed