Add CFDF_Document::GetMutableRoot()

Then update one caller.

Change-Id: Ie640d1bdd35a4f297b661f7abd1ca9e4e7621fe7
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/94754
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>

core/fpdfapi/parser/cfdf_document.h

diff --git a/core/fpdfapi/parser/cfdf_document.h b/core/fpdfapi/parser/cfdf_document.h
index cad94d0..e17ee91 100644
--- a/core/fpdfapi/parser/cfdf_document.h
+++ b/core/fpdfapi/parser/cfdf_document.h
@@ -26,7 +26,8 @@
   ~CFDF_Document() override;
 
   ByteString WriteToString() const;
-  CPDF_Dictionary* GetRoot() const { return m_pRootDict.Get(); }
+  const CPDF_Dictionary* GetRoot() const { return m_pRootDict.Get(); }
+  RetainPtr<CPDF_Dictionary> GetMutableRoot() const { return m_pRootDict; }
 
  private:
   void ParseStream(RetainPtr<IFX_SeekableReadStream> pFile);

core/fpdfdoc/cpdf_interactiveform.cpp

diff --git a/core/fpdfdoc/cpdf_interactiveform.cpp b/core/fpdfdoc/cpdf_interactiveform.cpp
index 810da94..cc2e271 100644
--- a/core/fpdfdoc/cpdf_interactiveform.cpp
+++ b/core/fpdfdoc/cpdf_interactiveform.cpp
@@ -935,7 +935,7 @@
     return nullptr;
 
   RetainPtr<CPDF_Dictionary> pMainDict =
-      pDoc->GetRoot()->GetMutableDictFor("FDF");
+      pDoc->GetMutableRoot()->GetMutableDictFor("FDF");
   if (!pdf_path.IsEmpty()) {
     auto pNewDict = pDoc->New<CPDF_Dictionary>();
     pNewDict->SetNewFor<CPDF_Name>("Type", "Filespec");