Constify the uniform value SkDatas in runtime effects

Bug: skia:12720
Change-Id: I7f74b4f88456bf5ddef1a648d2231d28034ed246
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/544237
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
6 files changed