Zero-initialize uniform data in SkRuntimeEffectBuilder

Change-Id: Id7445c28fca3a74cb2716832e821d651be7bb883
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/473142
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed