Require that runtime effect child variables are 'uniform'

Bug: skia:11374
Change-Id: I63d605eabbe514a0469d00d8a671969874f3edd4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/393081
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
5 files changed