Use ProgramVisitor in MetalCodeGenerator::requirements.

This boilerplate was written in 2017 before ProgramVisitor existed.

Change-Id: I63826cb16db4abaa3b02e2ad578a6337450ca843
Bug: skia:13662
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/571061
Reviewed-by: Arman Uguray <armansito@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
2 files changed