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/fpdfsdk/src/javascript/PublicMethods.cpp b/fpdfsdk/src/javascript/PublicMethods.cpp
index f510d51..f228072 100644
--- a/fpdfsdk/src/javascript/PublicMethods.cpp
+++ b/fpdfsdk/src/javascript/PublicMethods.cpp
@@ -242,7 +242,7 @@
     if (!FXSYS_iswdigit(c))
       break;
 
-    nRet = nRet * 10 + FXSYS_toDecimalDigitWide(c);
+    nRet = nRet * 10 + FXSYS_toDecimalDigit(c);
     nSkip = i - nStart + 1;
     if (nSkip >= nMaxStep)
       break;