Make down-conversion explicit from CFX_ByteString to CFX_ByteStringC.

core/fpdfapi/fpdf_parser/cpdf_parser.cpp

Index: core/fpdfapi/fpdf_parser/cpdf_parser.cpp
diff --git a/core/fpdfapi/fpdf_parser/cpdf_parser.cpp b/core/fpdfapi/fpdf_parser/cpdf_parser.cpp
index 305401525ce62ed28b27d020d5bb4a2e240a1a14..73ca71ef1a0ade19efa5fd954e3363ed22b2bb15 100644
--- a/core/fpdfapi/fpdf_parser/cpdf_parser.cpp
+++ b/core/fpdfapi/fpdf_parser/cpdf_parser.cpp
@@ -822,10 +822,11 @@ FX_BOOL CPDF_Parser::RebuildCrossRef() {
                           uint32_t dwObjNum =
                               pElement ? pElement->GetObjNum() : 0;
                           if (dwObjNum) {
-                            m_pTrailer->SetAtReference(key, m_pDocument,
-                                                       dwObjNum);
+                            m_pTrailer->SetAtReference(key.AsByteStringC(),
+                                                       m_pDocument, dwObjNum);
                           } else {
-                            m_pTrailer->SetAt(key, pElement->Clone());
+                            m_pTrailer->SetAt(key.AsByteStringC(),
+                                              pElement->Clone());
                           }
                         }
                       }