Make SkSL::ByteCodeFunction::preprocess thread-safe

Change-Id: I64a1731efb8d5923f81d2449ae2c27fda37fd0b6
Bug: skia:9427
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/242079
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
2 files changed