Without an `operator=` on our expressions and variables, we no longer
need to wrap all our expressions with a helper.
Change-Id: I8110079f61c9ad01997f7c4b376db223dc4b6e17
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/541063
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>