Update paint hinting for upcoming Skia API changes. (flutter/engine#6802)

engine/src/flutter/third_party/txt/src/txt/paragraph.cc

@@ -439,7 +439,7 @@ void Paragraph::Layout(double width, bool force) {
   paint.setAntiAlias(true);
   paint.setTextEncoding(SkPaint::kGlyphID_TextEncoding);
   paint.setSubpixelText(true);
-  paint.setHinting(SkPaint::kSlight_Hinting);
+  paint.setHinting(kSlight_SkFontHinting);
 
   records_.clear();
   line_heights_.clear();