Pass DSLModifiers by reference.

These objects are ~48 bytes apiece.
(Added bsalomon@ for public API review)

Change-Id: I41207929ec2c33caaf877d8530ad7c89b24b63cb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/436823
Commit-Queue: Brian Salomon <bsalomon@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
9 files changed