Rename sampleVariablesSupport() to sampleMaskSupport()

We have only implemented support for the sample mask. Renaming the cap
to reflect that.

Change-Id: I10b5923c7464093187336016963a8ac51ebd54f5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/249932
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
11 files changed