Convert FX_CHARSET to an enum type.

This is complicated because charsets are exposed as ints in the public
API, so some structs and functions must remain using ints.

Change-Id: I8136dc3a79f35483e87840e73d617594019fe4bf
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/83330
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/core/fxge/cfx_fontmapper.h b/core/fxge/cfx_fontmapper.h
index cd200c7..7d0ea7b 100644
--- a/core/fxge/cfx_fontmapper.h
+++ b/core/fxge/cfx_fontmapper.h
@@ -57,7 +57,7 @@
   }
 
   void SetSystemFontInfo(std::unique_ptr<SystemFontInfoIface> pFontInfo);
-  void AddInstalledFont(const ByteString& name, int charset);
+  void AddInstalledFont(const ByteString& name, FX_Charset charset);
   void LoadInstalledFonts();
 
   RetainPtr<CFX_Face> FindSubstFont(const ByteString& face_name,