Use FXSYS_IsDecimalDigit() in more places.

Also rename it from FXSYS_isDecimalDigit() and fix nits along the way.

Change-Id: Idce84dfa38ccc09e8b571c60f864e7d387c0040f
Reviewed-on: https://pdfium-review.googlesource.com/c/45751
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/core/fxcrt/fx_extension.h b/core/fxcrt/fx_extension.h
index d190431..e14c980 100644
--- a/core/fxcrt/fx_extension.h
+++ b/core/fxcrt/fx_extension.h
@@ -75,16 +75,16 @@
   return upchar > '9' ? upchar - 'A' + 10 : upchar - '0';
 }
 
-inline bool FXSYS_isDecimalDigit(char c) {
+inline bool FXSYS_IsDecimalDigit(char c) {
   return !((c & 0x80) || !std::isdigit(c));
 }
 
-inline bool FXSYS_isDecimalDigit(wchar_t c) {
+inline bool FXSYS_IsDecimalDigit(wchar_t c) {
   return !!std::iswdigit(c);
 }
 
 inline int FXSYS_DecimalCharToInt(char c) {
-  return FXSYS_isDecimalDigit(c) ? c - '0' : 0;
+  return FXSYS_IsDecimalDigit(c) ? c - '0' : 0;
 }
 
 inline int FXSYS_DecimalCharToInt(wchar_t c) {