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>
diff --git a/src/core/SkPaint_text.cpp b/src/core/SkPaint_text.cpp
index 7e2d2d2..33e9091 100644
--- a/src/core/SkPaint_text.cpp
+++ b/src/core/SkPaint_text.cpp
@@ -263,7 +263,7 @@
         sk_getAdvance_glyph_next,
     };
 
-    unsigned index = encoding;
+    unsigned index = static_cast<unsigned>(encoding);
 
     if (!needFullMetrics) {
         index += 4;