As can be seen in `src/core/fonts.js`, this method only accepts *one* parameter, hence it's somewhat difficult to understand what the Annotation-code is actually attempting to do here.
The only possible explanation that I can imagine, is that the intention was initially to call `Font.charToGlyph` *directly* instead. However, note that that'd would not actually have been correct, since that'd ignore one level of font-caching (see `this.charsCache`). Hence the unused arguments are removed, in `src/core/annotation.js`, and the `Font.charToGlyph` method is now marked as "private" as intended.