Added unary + and - DSL operators

These were accidentally omitted from the supported operator list.

Change-Id: Idecd17adb8b3f5043e36328c65ca12be33e990f4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/381637
Auto-Submit: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
3 files changed