Code change to avoid signed/unsigned mismatch warnings.

This makes pdfium code on Linux and Mac sign-compare warning free.
The warning flag will be re-enabled after checking on windows clang build.

BUG=pdfium:29
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1841643002 .
diff --git a/fpdfsdk/javascript/PublicMethods.cpp b/fpdfsdk/javascript/PublicMethods.cpp
index a4c1356..62659e4 100644
--- a/fpdfsdk/javascript/PublicMethods.cpp
+++ b/fpdfsdk/javascript/PublicMethods.cpp
@@ -1502,7 +1502,7 @@
   if (wChange.empty())
     return TRUE;
 
-  int iIndexMask = pEvent->SelStart();
+  size_t iIndexMask = pEvent->SelStart();
 
   size_t combined_len = wstrValue.length() + wChange.length() -
                         (pEvent->SelEnd() - pEvent->SelStart());