Make CPDF_Parser::GetRoot() return a const dictionary.

Astonishingly, no ripple effects.

Change-Id: I5eced72375d1cf35e66a5f3b513f10ac59b85cdb
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/94753
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/core/fpdfapi/parser/cpdf_parser.cpp b/core/fpdfapi/parser/cpdf_parser.cpp
index 14a9484..4a96e5a 100644
--- a/core/fpdfapi/parser/cpdf_parser.cpp
+++ b/core/fpdfapi/parser/cpdf_parser.cpp
@@ -852,10 +852,10 @@
   return GetTrailer() ? GetTrailer()->GetArrayFor("ID") : nullptr;
 }
 
-CPDF_Dictionary* CPDF_Parser::GetRoot() const {
+const CPDF_Dictionary* CPDF_Parser::GetRoot() const {
   CPDF_Object* obj =
       m_pObjectsHolder->GetOrParseIndirectObject(GetRootObjNum());
-  return obj ? obj->GetMutableDict().Get() : nullptr;
+  return obj ? obj->GetDict() : nullptr;
 }
 
 const CPDF_Dictionary* CPDF_Parser::GetEncryptDict() const {
diff --git a/core/fpdfapi/parser/cpdf_parser.h b/core/fpdfapi/parser/cpdf_parser.h
index 094e1dd..559850e 100644
--- a/core/fpdfapi/parser/cpdf_parser.h
+++ b/core/fpdfapi/parser/cpdf_parser.h
@@ -89,7 +89,7 @@
   uint32_t GetRootObjNum() const;
   uint32_t GetInfoObjNum() const;
   const CPDF_Array* GetIDArray() const;
-  CPDF_Dictionary* GetRoot() const;
+  const CPDF_Dictionary* GetRoot() const;
 
   const CPDF_Dictionary* GetEncryptDict() const;