Convert FWL_ERR into an enum class.

xfa/fwl/core/ifwl_app.h

Index: xfa/fwl/core/ifwl_app.h
diff --git a/xfa/fwl/core/ifwl_app.h b/xfa/fwl/core/ifwl_app.h
index 53dc35d33ab9f386d0a358506579416874445af0..477c0f2d196fea26cefe60a4f2f90b61cc4bcf40 100644
--- a/xfa/fwl/core/ifwl_app.h
+++ b/xfa/fwl/core/ifwl_app.h
@@ -34,13 +34,13 @@ class IFWL_App {
 
   virtual ~IFWL_App() {}
 
-  FWL_ERR Initialize();
-  FWL_ERR Finalize();
+  FWL_Error Initialize();
+  FWL_Error Finalize();
   IFWL_AdapterNative* GetAdapterNative();
   IFWL_WidgetMgr* GetWidgetMgr();
   IFWL_ThemeProvider* GetThemeProvider();
-  FWL_ERR SetThemeProvider(IFWL_ThemeProvider* pThemeProvider);
-  FWL_ERR Exit(int32_t iExitCode);
+  FWL_Error SetThemeProvider(IFWL_ThemeProvider* pThemeProvider);
+  FWL_Error Exit(int32_t iExitCode);
 
   // These call into polymorphic methods in the impl; no need to override.
   void Release();