return self from matrix setters to allow chaining

Change-Id: Ifd4c6bf5f5feb925f06067d50e491fd1a490f896
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/228344
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Auto-Submit: Mike Reed <reed@google.com>
3 files changed