Support expressions for scalar values

Change-Id: I87c1ca4dd6d6f2e1e591eedc97ee7cb088039eab
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/436117
Commit-Queue: Avinash Parchuri <aparchur@google.com>
Auto-Submit: Kyle Carlstrom <kylecarlstrom@google.com>
Reviewed-by: Avinash Parchuri <aparchur@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
13 files changed