Use size_t for lengths in StringTo().

As would be flagged by -Wshorten-64-to-32

Change-Id: Idcd889acb0103514d555560b75399a8ba69cb902
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/86895
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/core/fxcrt/fx_string.cpp b/core/fxcrt/fx_string.cpp
index 172b196..9b86abb 100644
--- a/core/fxcrt/fx_string.cpp
+++ b/core/fxcrt/fx_string.cpp
@@ -49,9 +49,9 @@
   if (strc.IsEmpty())
     return 0;
 
-  int cc = 0;
   bool bNegative = false;
-  int len = strc.GetLength();
+  size_t cc = 0;
+  size_t len = strc.GetLength();
   if (strc[0] == '+') {
     cc++;
   } else if (strc[0] == '-') {