Rename domain types and vars to subset.

Makes nomenclature more conistent across different classes.

Change-Id: I9f052bbd38082d95714702b2ae960c4e15fdc181
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/284718
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
8 files changed