Fix some sign comparison warnings.

fpdfsdk/src/javascript/Document.cpp

Index: fpdfsdk/src/javascript/Document.cpp
diff --git a/fpdfsdk/src/javascript/Document.cpp b/fpdfsdk/src/javascript/Document.cpp
index 7ce8449f3cb21df005950ade9f2c2c36847eb215..a8797095d81b10d4650cec165306371e54d107aa 100644
--- a/fpdfsdk/src/javascript/Document.cpp
+++ b/fpdfsdk/src/javascript/Document.cpp
@@ -1201,7 +1201,7 @@ CFX_WideString Document::CutString(CFX_WideString cbFrom) {
   wchar_t* pResult = (wchar_t*)malloc(iSize.ValueOrDie());
   wchar_t* pFrom = (wchar_t*)cbFrom.GetBuffer(iLength);
 
-  for (int i = 0; i < iLength; i++) {
+  for (size_t i = 0; i < iLength; i++) {
     if (pFrom[i] == L'\\' || pFrom[i] == L'/') {
       pResult[i] = L'\0';
       break;