Update GrAtlasTextOp::MaskType to enum class

Change-Id: I16a5938f5156fac2e93b8cd0b2a8e2ed45b46386
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/330938
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
3 files changed