change SkTextEncoding to enum class

Bug: skia:
Change-Id: I3ea2792a49ca93fdd4bba686c63047727372f88b
Reviewed-on: https://skia-review.googlesource.com/c/170880
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
2 files changed