Fix for-loop which results in no-op

The existing code (added in https://codereview.chromium.org/1650793002/)
changes the state of a copy of the object.

Original CL: https://codereview.chromium.org/1679783002/
Original Author: hashimoto@chromium.org

R=hashimoto@chromium.org

Review URL: https://codereview.chromium.org/1681863002 .
diff --git a/core/src/fpdfapi/fpdf_parser/fpdf_parser_parser.cpp b/core/src/fpdfapi/fpdf_parser/fpdf_parser_parser.cpp
index f170b20..9d646b2 100644
--- a/core/src/fpdfapi/fpdf_parser/fpdf_parser_parser.cpp
+++ b/core/src/fpdfapi/fpdf_parser/fpdf_parser_parser.cpp
@@ -973,7 +973,7 @@
   if (bMainXRef) {
     m_pTrailer = ToDictionary(pStream->GetDict()->Clone());
     ShrinkObjectMap(size);
-    for (auto it : m_ObjectInfo)
+    for (auto& it : m_ObjectInfo)
       it.second.type = 0;
   } else {
     m_Trailers.Add(ToDictionary(pStream->GetDict()->Clone()));