make GrTextBlob::addMultiMaskFormat private

Change-Id: Ie3592487d18a719f5ca6a226c971e30c2c01c139
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/374359
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed