Combine StrToInt methods.

This Cl combines the two StrToInt implementations. In doing so I had to add
some more overrides to toDecimalDigit() and add a isDecimalDigit().

BUG=pdfium:423
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1757283002 .
diff --git a/core/include/fxcrt/fx_ext.h b/core/include/fxcrt/fx_ext.h
index 3f4f668..c8afb2b 100644
--- a/core/include/fxcrt/fx_ext.h
+++ b/core/include/fxcrt/fx_ext.h
@@ -53,13 +53,21 @@
   return upchar > '9' ? upchar - 'A' + 10 : upchar - '0';
 }
 
+inline bool FXSYS_isDecimalDigit(const FX_CHAR c) {
+  return std::isdigit(c);
+}
+
+inline bool FXSYS_isDecimalDigit(const FX_WCHAR c) {
+  return std::iswdigit(c);
+}
+
 inline int FXSYS_toDecimalDigit(const FX_CHAR c) {
   if (!std::isdigit(c))
     return 0;
   return c - '0';
 }
 
-inline int FXSYS_toDecimalDigitWide(const FX_WCHAR c) {
+inline int FXSYS_toDecimalDigit(const FX_WCHAR c) {
   if (!std::iswdigit(c))
     return 0;
   return c - L'0';