Move invalid uniform protection to the frontend. The frontend potentialy indexes into mUniformLocations with invalid uniform locations while validation is disabled too. Move the validation from the Metal backend to the frontend. Bug: chromium:1484878 Change-Id: I92bc43aa28cfa26d601bb28f318860375f618608 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4947652 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>