Bug 849866 - fix signed/unsigned comparison warning in nsDocument::Register; r=smaug

content/base/src/nsDocument.cpp

@@ -5028,7 +5028,7 @@ nsDocument::Register(JSContext* aCx, const nsAString& aName,
 
   // Do element upgrade.
   nsRefPtr<nsContentList> list = GetElementsByTagName(lcName);
-  for (int32_t i = 0; i < list->Length(false); i++) {
+  for (uint32_t i = 0; i < list->Length(false); i++) {
     nsCOMPtr<nsINode> oldNode = list->Item(i, false);
 
     // TODO(wchen): Perform upgrade on Shadow DOM when implemented.