Reduce signed/unsigned comparison warnings

The warnings generated by Clang. This is part 1 for some simple cases.

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

Review URL: https://codereview.chromium.org/1840483003 .
diff --git a/fpdfsdk/javascript/JS_Value.cpp b/fpdfsdk/javascript/JS_Value.cpp
index 1d57833..4d2a1d4 100644
--- a/fpdfsdk/javascript/JS_Value.cpp
+++ b/fpdfsdk/javascript/JS_Value.cpp
@@ -893,7 +893,7 @@
 
   va_list ap;
   va_start(ap, nKeywords);
-  for (int i = 0; i < nKeywords; ++i) {
+  for (size_t i = 0; i < nKeywords; ++i) {
     const wchar_t* property = va_arg(ap, const wchar_t*);
     v8::Local<v8::Value> v8Value =
         FXJS_GetObjectElement(pRuntime->GetIsolate(), pObj, property);